Updating `get_distances` function of `seqstats`.

Updating `get_distances` function of `seqstats` module to handle
amino acid sequences. If sequences are passed that cannot be
reverse complemented, the reverse-complement distance is returned
as `None`.

Adding tests to `test_seqstats.py`.
Updating `ResidueAlphabet`; adding `RnaAlphabet`.

Adding `has_state` method to `ResidueAlphabet` class, and adding
`RnaAlphabet` class.
Updating `summarize_distances` function.

Updating `summarize_distances` function of `seqsum` module to handle
`None` values returned for the reverse-complement distances from
distance functions.