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.
Thanks! I missed that one.  Just the same though, the possible introduction of the ```PurinePyrimidineSequence``` subclass notwithstanding, I think there is still a use case for interrogating a ```NucleotideSequence``` with something like the ```find_features``` method posed here (e.g., if you wanted to find the purine/pyrimidine runs but wanted not only the length of the run, but also the start/stop positions of the run and the actual nucleotides -- in the original alphabet -- that compose the run).