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
Merge branch 'master' of github.com:biocore/bipy into issue-25
reorg tests as described in #48