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.
I believe you are correct for the most part, and when I think about it, I come to the conclusion that the complexity of features that this particular method should be able to identify is described very well by CFGs (and, therefore, regular expressions).  Allowing ```NucleotideSequence``` (or ```BiologicalSequence```) objects to identify more complex features would probably require a set of functions too heterogeneous in their signatures to provide efficient implementations as part of a single ```find_features``` function.