Add retrieve_data() to get data through a URL call.

    Steps ::
        If hashes provided:
            1. Check hash table for data file.
            2. Check hash subdirectory within cache directory for data file.
            3. If data file not in cache, download file, compute hash,
               and verify hash.
            4. If hash correct, save file.
        Otherwise, simply download file.
Merge branch 'master' of github.com:binarybottle/mindboggle