Adding alphabet classes.

Adding `ResidueAlphabet` and `DnaAlphabet` classes to new
`seqsift.utils.alphabets` modules. This moves the DNA ambiguity code
handling from globals initiated in the utils `__init__.py` to the
`DnaAlphabet` class.

This change required changes to the `align` module, because the old dna
ambiguity global was used in the `align` module's functions.

Adding tests for new classes to `test_alphabets.py`.
All package tests are currently passing.
Adding `.gitignore`.
Starting work on new `seqstats` function.

Adding `get_differences`, which will be a function for documenting
state differences between two sequences. It currently is only
a place holder and is `pass`ing.