addition of python functions for pairwise local and global alignment 

Ready for review for merge. I'll likely make some behind-the-scenes changes in the future, like possibly creating an ``Aligner`` class, but that shouldn't affect interface so I'd like to do in a separate PR. The goal of this one is to get usable functionality in place. @ebolyen, it'd be good to have you review the interface for consistency with the SSW code. Also, I think we'll likely end up with a merge conflict in the docs in ``alignment/__init__.py``, but that should be easy to address. 

~~**NOT READY FOR MERGE**~~

Can you preserve the original IDs of the sequences (if they were `BiologicalSequence` instances) and use these IDs in the alignment?

Since sequence IDs are optional, it may make more sense to omit them from the alignment if `seq1` and `seq2` are strings (instead of hardcoding `0` and `1`).