ENH: Add iter_regex and find_features

Fixes #612. Adds iter_regex to skbio._sequence.BiologicalSequence and
find_features to skbio._sequence.NucleotideSequence. Supported features are
currently purine_run and pyrimidine run, but more can be added by specifying
additional regular expressions.
Maybe I misunderstand, but aren't HHMs just a stochastic FA, or rather an NFA with edge weights? I don't think I understand how that could be more computationally powerful than a CFG. If we want to support complex feature detection, it might be worthwhile to explore (S)CFG evaluation frameworks. 

The one thing I definitely appreciate about a regular expression vs an automoton (and this holds for CFG's vs a tokenizer/lexor/parser trio) is a declaritive syntax vs an implementation. By describing the language, we only need validation (in the software engineering sense), wheras an implementation needs both validation and verification.

I agree that the numpy array is a good idea and switching gives us some powerful options, but it is ultimately an implementation detail and is discrete from our interface.