fixes #54
Merge pull request #55 from gregcaporaso/issue-54

adds exception.py file
STY: Fix pep8 failure.
Merge pull request #56 from Jorge-C/master

STY: Fix pep8 failure.
Merge branch 'master' of github.com:biocore/bipy into issue-25
updated to import BiologicalSequenceError
updated to import BiologicalSequenceError