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.
Oh, I missed that discussion, sorry, can you link me to it?  I didn't see it in #612 or the [original stack overflow post](http://stackoverflow.com/questions/25211905/determine-length-of-polypurine-tract) that led to the creation of that issue.  What would a ```PurinePyrimidineSequence``` be?  Isn't a nucleotide sequence just a sequence of purines and pyrimidines?