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.
Casting ```numpy.array``` to ```str``` is on the order of microseconds for character arrays of length one million, so if/when we change this to numpy arrays, we can add a cast to ```str```