Coveralls support

### Motivation
Add coveralls support. See #622 for more details. This will help us improve our currently minuscule testing coverage.

### Changelog
* Include coveralls dependencies in requirements
* Hook travis with coveralls
* Include a shameful badge on README
* Update Makefile to run tests with coverage report
* Remove failing feature test which breaks the build

### Usage guide
Coveralls will report the coverage of your pull requests automatically. You should create a [coveralls](https://coveralls.io/) account and enable coveralls on your own repo fork on GitHub (after enabling travis, of course) so that you can see the coverage of pushes as they go through.

On the coveralls website, you can see the coverage of individual files. This will help you improve your tests. See for example [our db_store test coverage](https://coveralls.io/files/297925580).