Add support for user defined exception translators

Skip eigen test when eigen header available but NumPy not

This allows (and changes the current examples) to exit with status 99 to
skip a test instead of outputting a special string ("NumPy missing").

This also fixes the eigen test, which currently fails when eigen
headers are available but NumPy is not, to skip instead of failing when
NumPy isn't available.

Fix gcc6 placement new warning

Otherwise this would create unknown option warnings under g++ < 6.

GCC-6 adds a -Wplacement-new warning that warns for placement-new into a
space that is too small, which is sometimes being triggered here (e.g.
example5 always generates the warning under g++-6).  It's a false
warning, however: the line immediately before just checked the size, and
so this line is never going to actually be reached in the cases where
the GCC warning is being triggered.

This localizes the warning disabling just to this one spot as there are
other placement-new uses in pybind11 where this warning could warn about
legitimate future problems.

 Add _<bool>("s1", "s2") ternary & use TYPE_CASTER

This commit adds an additional _ template function for compile-time
selection between two description strings.  This in turn allows the
elimination of needing two name() methods in type_caster<arithmetic
types> and type_caster<eigen types>, which allows them to start using
PYBIND11_TYPE_CASTER instead, simplifying their code by eliminating all
the code that they are duplicating from the macro.

Tighten check() test in eigen.py

Previous version would give false 'OK' if, for example, we were supposed
to get [1, 2, 3] but instead got [2, 1, 3].

Fails --- next commit will tighten test.

Non-contiguous arrays

Add and declare to Python functions

    double_mat_cm() --- compute 2* a column-major matrix
    double_mat_rm() --- compute 2* a row-major matrix

to 'eigen.cpp' tests / example.

In eigen.h, type_caster<Type>::load():  For the 'ndim == 1' case, use
the 'InnerStride' type because there is only an inner stride for a
vector.  Choose between (n_elts x 1) or (1 x n_elts) according to
whether we're constructing a Vector or a RowVector.

Passing a non-contiguous one-dimensional numpy array gives incorrect
results, so three of these tests fail.  The only one passing is the
simple case where the numpy array is contiguous and we are building a
column-major vector.  Subsequent commit will fix the three failing
cases.

Fix test diff output under python2.7

PR #220 broke failed test output under python2.7, which doesn't support
the keepends argument to splitlines.

More informative error message when `py::make_tuple` fails

Allow std::enable_shared_from_this when using const element type

Allows const types to be used by shared_ptr