Refactor `coverage` task.

It's simpler now. Instead of setting branch/source/omit in our custom code, it's recommended to use a `.coveragerc` file.
This will make it easier to "discover" where coverage settings are set and will work no matter how Coverage is run.
Merge pull request #15 from joncotton/coverage_refactor

Use `.coveragerc` and refactor `coverage` task