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
temporary fix for #691
changed required version, based on suggestion from @jairideout
updating travis to restrict future version
fixed typo
Merge pull request #692 from gregcaporaso/issue-691

temporary fix for #691
BUG: fixed bug that resulted from change in biom API
TST: fixed test to support change
Merge pull request #693 from gregcaporaso/issue-641

BUG: fixed bug that resulted from change in biom API
Make equal sequence length an invariant of Alignment.
Flake8 changes.
Updated Alignement.is_valid() doctest.
Replace future.utils.six with six imports
Add six as a required dependency; update py2/3 doc

Fixes #691.
Merge pull request #696 from jairideout/issue-691

Change future.utils.six imports to direct six imports: redux
Remove skbio deps from readme