Add coveralls support

Use http://coveralls.io to measure code coverage. This gives several nice features:
 * View and compare coverage for existing code with handy code browser. This helps us to identify untested code
 * Check PRs code coverage. 'Changed' tab on coveralls will show if all the added code is being covered with tests. Coverall comments the PR with coverage report

Sample links:
 * [Coverage progress for my behave repo](https://coveralls.io/r/roignac/behave)
 * [Coverage report for PR](https://coveralls.io/builds/154887) and [sample comment](https://github.com/roignac/behave/pull/3#issuecomment-22456979)
 
Actions required:
 * Register the repo at http://coveralls.io
 * Merge the PR
 * Put a badge in README.md

Note, that this change also does the following:
 * Disables parallel nosetest - this confuses coveralls and doesn't really speed up tests
 * Uses a nasty coverage-enabled run of self-test features in .travis.yml - this should be replaced with a nice macro, but I'm not sure how this is supposed to be implemented