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.
Simplify the regex considerably

Not quite sure what I was thinking with that one
Add option to allow gaps
Move feature types into class hierarchy

Also add some additional tests
Merge branch 'master' of https://github.com/biocore/scikit-bio into issue-612

Conflicts:
	skbio/sequence/tests/test_sequence.py
Change import of re

Thanks @ebolyen and @jairideout
Merge pull request #613 from adamrp/issue-612

ENH: Add iter_regex and find_features