Allow std::enable_shared_from_this when using const element type

Allows const types to be used by shared_ptr

Added warning about same-address-optimization

See https://github.com/pybind/pybind11/issues/254

Use prefix increment in make_iterator

Add callback examples with named parameters

Allow pybind11::arg to have 0, false, or "" as default values.

Skip FindPythonLibsNew if PYTHONLIBS_FOUND is defined (fix #236)

Also:
- Update documented result variables
- Remove long-ago-deprecated variable

Check the number of named arguments at compile time

Syntax sugar: user-defined literals for arg and str