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 think we don't have a standard but we suggest people use whatever the
community uses. That being said I've seen both `from re import compile
...`.

On (Oct-07-14|14:54), Evan Bolyen wrote:
> > @@ -9,6 +9,7 @@
> >  from __future__ import absolute_import, division, print_function
> >  from future.utils.six import string_types
> >  
> > +from re import compile as re_compile
> 
> This might make more sense as `import re` then it becomes `re.compile`. Though I don't know if we have standards for this to indicate the use of one over the other.
> 
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