Add coverage report

@opyate @tsangsimon  can one of you guys please take a look. Coverage using pytest-cov is added. I'm now trying to get this synced up to a service called coveralls to show the report somewhere

To run the tests locally, activate the virtualenv  for the project. install all requirements (prod and test) then run

py.test --cov searchapi tests/ --cov-report=term --cov-report=html