Added to_file method for StockholmAlignment and test for it #495

Can you please update this to support file paths and file handles? See [`DissimilarityMatrix.to_file`](https://github.com/biocore/scikit-bio/blob/master/skbio/core/distance.py#L681) for an example. See [this test case](https://github.com/biocore/scikit-bio/blob/master/skbio/core/tests/test_distance.py#L181) for an example of how to test both types of input.