MAINT: Move numpy/core/code_generators/cversions.py to numpy/core.

This is done because it runs as a script, hence without context, and
this causes problems with imports in the code_generators package. This
became apparent when numpy_api.py was changed to import genapi.py from
the same package.
DOC: Document the new location of cversions.py.

The instructions in HOWTO_RELEASE.rst.txt needed to be modified.
MAINT: Update the C-API hash in cversions.txt.

The new annotations caused the api hash to change.
MAINT: Delete code_generators/cversions.py in its old location.
MAINT: Add NPY_1_9_API_VERSION to numpyconfig.h.

Defined to the same value it same value as the NPY_1_8_API_VERSION as
there have been no deprecated macros or functions in the include files.
DOC: Update HOWTO_RELEASE.rst.txt.

Add more detail in the "Check the C API version number" section.
Merge pull request #4670 from charris/prepare-for-1.9-branch

Prepare for 1.9 branch