MAINT: Refactor tests.

See discussion in https://github.com/biocore/bipy/issues/48
MAINT: Add `__init__` files to all tests folders for consistency.
MAINT: Update travis.yml to new package structure.
Merge pull request #50 from Jorge-C/test_reorg

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

adds exception.py file
STY: Fix pep8 failure.