Introduce `python-coveralls` dependency

Register `coveralls` for `after_success` hook in Travis CI.
Add coverage badge to `README.md`
Merge pull request #5 from 0x64746b/feature/use_coveralls

Integrate https://coveralls.io
Add 2 tests for divisions
Merge remote-tracking branch 'origin/master' into feature/add_tests

Conflicts:
	requirements/tests.txt