Implement hdf Matrix storage wrapper

[thcrock]

There isn't a usable storage wrapper for hdf5 matrices. We use the MettaCSVMatrixStore in experiments: https://github.com/dssg/catwalk/blob/master/catwalk/storage.py#L170

But in order to work with hdf5 files, we need to create a similar storage wrapper. The never-before-used MettaMatrixStore in catwalk.storage is close, but still needs to implement the 'empty' and 'columns' methods.

[thcrock]

Was addressed in #24