support from buffer memory files

I'm a heavy user of file images and this was not currently in h5py so I've added it. I did not know cython very well (little documentation where I needed it) so I had to spend a few hours figuring out how best to get buffer memory and figure out exactly in what layer of functions it had to go.  I may have gotten that wrong but this is working for me.  I will submit a pull request for master separately.