HDF5 Support

[bearpaw]

Thanks for this amazing library. I was wondering for large datasets with millions of images, would it make sense to cache in a single file (e.g., HDF5) instead of creating millions of cache files? Do you have any plan to support Hdf5 format? Thanks!

[szymonmaszke]

Will check out HDF5 and other formats and update for upcoming 0.3.0 release. If you have any suggestions feel free to post, thanks.