100% coverage

Achieved 100% test coverage.  Current PR also requires 100% coverage to pass tests.

`functoolz.hashable` was removed (see #75) because it is never used.

Note that coverage checks can be skipped by adding a comment `# pragma: no cover`.  This was only done in `compatibility` due to different python versions executing different code.

This is related to #70, which is about showing coverage stats using https://coveralls.io/