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.

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

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