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**~~

Would be better to use `npt.assert_warns` in the specific test cases where warnings are raised. This will ensure that a warning is raised, allow you to compare observed to expected output, and avoid masking warnings that we might want to know about if other tests are added in the future. See tests for `skbio.draw.distributions` for an example (e.g., [here](https://github.com/biocore/scikit-bio/blob/master/skbio/draw/tests/test_distributions.py#L169)).