MAINT: Remove doc/numpybook.

Remove doc/numpybook. It has become outdated in parts and its contents
is now part of the official numpy documentation.
DOC: Document removal of doc/numpybook in 1.9.0 release notes.
DOC: Change pyrex to Cython in c-info.how-to-extend.rst.
WIP: Changing np.fromfile handling to Python API
Revert "WIP: Changing np.fromfile handling to Python API"

This reverts commit 71e8e356ed0378bb121073afa8671ad5af047958.