automate version number, fix warnings, add function H5DOwrite_chunk

Since 2.3 there were really grat work on h5py ! Great job !
Thanks for all the changes, even if my pull requests were rejected.
Neverless here I have 2 commits that could enhance some features and remove compile warnings.
- I added the ./c2py.py which helps to keep the api_functions.txt and api_types_hdf5.pxd up to date 
- cython now supports const, and I updated h5py to this.
- function H5DOwrite_chunk was added

Please take what you find usefull and reject the rest.
Regards
 Thierry