Adding `ProteinAlphabet` class to `alphabets`.
Adding amino acid tests of `get_differences`.

Adding tests of `get_differences` function (of `seqstats` module) with
amino acid sequences.
Updating `seqvet.py` script.

importing `alphabets` module and setting `alphabet` variable to a
residue alphabet according to `data-type` option. This is now passed to
`summarize_distances` function.