Integrate with coveralls coverage reporting
Only report coverage on unit tests.
Send coverage report straight after unit tests
Omit virtualenv from coverage reporting.
Omit virtualenv from coverage report.
Make pip install quiet in travis config.

The progress indicator produces a lot of noise in the travis output.
Also, the travis UI breaks down if the output gets too long.
Fix travis coverage integration

- Put the correct script in the after_script section
- Limit the coverage report to the backdrop package
Remove --verbose flag from coveralls call

It produces masses of output that the travis web interface has
difficulty handling.
Move coverage config to .coveragerc

Allows reporting config to be picked up by coveralls.