Feature/coverage

These are the changes required in velocity for the coverage feature to work. They are all around the default-fixture and it being copied. 

I also apologize for not submitting a pull request for my previous changes that are also needed for coverage, I was using my IDE and didn't realize I was working on master and hadn't squashed my commits either. Thankfully nothing happened but please also review those changes as part of this pull-request and provide any feedback. Here are the commits of interest:

[fix for all frameworks completion detection](https://github.com/xolvio/velocity/commit/8b56e11d3b2a84aa52e408dacbc3a224fa625763)

[Added preprocessors, reporters and a VelocityMirrors collection](https://github.com/xolvio/velocity/commit/283a00a3236f92fca952d26fca9a864836c8e0b7#diff-d41d8cd98f00b204e9800998ecf8427e)

I extracted the coverage feature to a separate package which can be [seen here](https://github.com/xolvio/velocity-coverage)


Maybe we could come up with another name for these reporters. At first I thought that the html-reporter would be such a reporter but it seems different since these reporters run after all the tests have finished.