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
DOC: Added missing IPython hyperlink to README.
Merge pull request #683 from Kleptobismol/docfix

DOC: Added missing IPython hyperlink to README.
WEB: new css for inline code blocks
Merge pull request #687 from ebolyen/code_style

WEB: new css for inline code blocks
DOC: added more explanation to top level package and added supported objects for each format
DOC: explained reserved kwargs in IO
DOC: explained compound formats
DOC: explained filehandle mutation for each registry operation
DOC: added comments to registry
ENH: empty file is now explicitly checked
DOC/STY: pep8 and note on empty sniffer
DOC: changed some language, thanks @johnchase
DOC: Fixed some language and added unit-testing section, thanks @jairideout
DOC: better introduction by @jairideout
DOC: fixed links