expand doc section on cython -a output

* explain new "+" signs in front of lines
* explain different shades of yellow
* relation to nogil blocks

Fix caching issues of cython magic.

Should fix https://github.com/ipython/ipython/issues/6507 when using cython magic. 
We will deprecate cython magic in IPython itself.

improve safety of cname substring replacements for C++ string conversion functions: only replace first occurrence as there may be a user defined portion following

undo C++ to-py helper functions rewrite as it lost the ability to extend the set of string-ishly convertible types

reduce C code overhead for C++ std::string -> PyObject conversion helpers by implementing them directly in C++ and not Cython

--HG--
extra : transplant_source : %3E%C7%7C%E0I%E9%84%03%8A%EF%F0z%84%20d%E9%C1%12%AF%3E

DOC initialize string with bytes

As-is the example doesn't compile.

fix: not setting module reference to NULL in module init function fails to report exception on return

docs: mention that static typing with Py3 signature annotations requires the "annotation_typing=True" directive

Fix crash when an exception occurs during module init and the stale module gets used later on (after import failure).
This is fixed by no longer cleaning up the global references to module object and module dict, which obviously has the drawback of leaking memory.

provide "Cython.Compiler.Main.Version" to keep supporting old PyTables versions that import it from there

--HG--
extra : transplant_source : %B1%BC%5C%CD%A6%EEmr4B%0F%AF%1C%E0yq9%EA%ADX