Adds .coveragerc and modify tox.ini for coverage

Adds some default settings to coverage tool via .coveragerc config file
Adds erase and report commands to cover env in tox.ini
Adds coveralls.io config

Adds coveralls to test-requirements.txt
Adds coveralls to after_success section in .travis.yml
Adds coveralls badge to README.rst
Merge pull request #4 from petrblaho/test-coverage

Test coverage reporting setup