[Docs][Ready for Review] Added documentation on extension class instanciation from existing pointer

Fix error reporting on very early errors when the Cython runtime configuration module is not set up yet.
See #2199.

Made code examples in the docs compatible with python 2 and 3.

See #2279.

Fixing c library tutorial

Added a C++ example in the cython tutorial.

Make non-standard Extension attributes "py_limited_api" and "np_pythran" optional for cycache fingerprinting.

Added a documentation for the Cythonize function and its arguments.

Fixed the formatting of a note in special_methods.rst.

Replaced the text in reference/extension_types.rst by links to the corresponding sections in the userguide.

Transfer of the dynamic attributes into the userguide.

Replaced the text in reference/extension_types.rst by links to the corresponding sections in the userguide.

Transfered a note from the reference guide extension types to the userguide.

Also reject conflicting duplicates of top-level directives.
Closes #2230.

Support more C file extensions when searching modules during coverage analysis: .cc and .cxx in addition to the current .c and .cpp.
Closes #2266.