travis: add support for coveralls.io
README: add coverage badge
add coverage workaround

Add the workaround documented in
https://github.com/coagulant/coveralls-python#nosetests
rename .coverage.rc -> .coveragerc

Goodness knows why I named it incorrectly.
.coveragerc: add tests to omitted files
Merge pull request #46 from rjw57/travis-coverage

add coverage information to README