Currently pybind11 doesn't check when you define a new object (e.g. a
class, function, or exception) that overwrites an existing one.  If the
thing being overwritten is a class, this leads to a segfault (because
pybind still thinks the type is defined, even though Python no longer
has the type).  In other cases this is harmless (e.g. replacing a
function with an exception), but even in that case it's most likely a
bug.

This code doesn't prevent you from actively doing something harmful,
like deliberately overwriting a previous definition, but detects
overwriting with a run-time error if it occurs in the standard
class/function/exception/def registration interfaces.

All of the additions are in non-template code; the result is actually a
tiny decrease in .so size compared to master without the new test code
(977304 to 977272 bytes), and about 4K higher with the new tests.

NumPy scalars to ctypes conversion support

This reduces direct access to internals.registered_types_cpp to
just a few places.

This avoid a hashmap lookup since the pointer to the list of
direct converters is now cached in the typeinfo.

Bugfix: bad delete if no copy ctor

* Remove obsolete example reference
* Make example fully-working (except for #includes)

Fixes #456.

Fix def_property and related functions

Making `cppfunction` explicit broke `def_property` and friends.
The added tests would not compile without an implicit `cppfunction`.

Without the previous commit, this test generates a core dump.

type_caster_generic::cast(): The values of

    wrapper->value
    wrapper->owned

are incorrect in the case that a return value policy of 'copy' is
requested but there is no copy-constructor.  (Similarly 'move'.)  In
particular, if the source object is a static instance, the destructor of
the 'object' 'inst' leads to class_::dealloc() which incorrectly
attempts to 'delete' the static instance.

This commit re-arranges the code to be clearer as to what the values of
'value' and 'owned' should be in the various cases.  Behaviour is
different to previous code only in two situations:

policy = copy but no copy-ctor: Old code leaves 'value = src, owned =
true', which leads to trouble.  New code leaves 'value = nullptr, owned
= false', which is correct.

policy = move but no move- or copy-ctor: old code leaves 'value = src,
owned = true', which leads to trouble.  New code leaves 'value =
nullptr, owned = false', which is correct.

Support std::shared_ptr holder type out of the box

Reorganize documentation

With this there is no more need for manual user declarations like
`PYBIND11_DECLARE_HOLDER_TYPE(T, std::shared_ptr<T>)`. Existing ones
will still compile without error -- they will just be ignored silently.

Resolves #446.

Auto-implement format/numpy descriptors for enum types

Make operator bool() explicit

This prevents unwanted conversions to bool or int such as:
```
py::object my_object;

std::cout << my_object << std::endl; // compiles and prints 0 or 1
int n = my_object; // compiles and is nonsense
```

With `explicit operator bool()` the above cases become compiler errors.

Disable most implicit conversion constructors

We have various classes that have non-explicit constructors that accept
a single argument, which is implicitly making them implicitly
convertible from the argument.  In a few cases, this is desirable (e.g.
implicit conversion of std::string to py::str, or conversion of double
to py::float_); in many others, however, it is unintended (e.g. implicit
conversion of size_t to some pre-declared py::array_t<T> type).

This disables most of the unwanted implicit conversions by marking them
`explicit`, and comments the ones that are deliberately left implicit.

Accept any sequence type as std::vector (or std::list)

Accept any sequence type as std::vector

Fix dynamic attribute inheritance in C++

`PyType_Ready` would usually perform the inheritance for us, but it
can't adjust `tp_basicsize` appropriately.

Add dynamic attribute support