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.
@wasade Exactly, if we did not approach the problem from an implementation view and instead interpreted a declaritive grammar, `_sequence` would never need to be passed, and its datatype would be an implementation detail.