Writing Clustal output

Uploading code to write Clustal output as requested by "Port clustal I/O to skbio.io #654"
I know this was just a part of the original code, but if you wouldn't mind removing each docstring and putting the meaning into the function name. (It makes it show up better in nose). i.e. `def test_null` would becom `def test_clustal_to_generator_with_empty_input`.