Reset previous test reports when starting a new run

Fixes #68 – the issue with **duplicated** test reports in velocity, caused by the lack of consistent IDs for test cases. This simply resets all previous reports (removes them from the db) before a new test run. Now its possible to rerun the tests as often as you want.

I found no easy way to get a consistent ID for a test result from mocha. Jasmine has this, but I think this solution is equally great. I can't think of a scenario where overwriting (upserting) old test results is better than removing the old ones and writing them again.