Fixes #38 and fixes #39

@Sanjo & @rissem 
This branch now uses freeport. This PR is ready to go I think as is because [I've made it](https://github.com/xolvio/velocity/compare/feature/mirror-service#diff-7a9076d6d94e62c13d641aa71f19ae8eR564) still start a mirror on port 5000. It doesn't yet wait for the app to be up, but that's another feature to do (#58)

@Sanjo 
Once you implement that changes we spoke about in the FileCopier to also deal with files in the Fixture collection, then you'll get a [velocityResetDatabase](https://github.com/xolvio/velocity/compare/feature/mirror-service#diff-b94a7600642971149cf4170c6c2cf7e8R18) method. I've not made a clear collection method as this is ultrafast and I think its enough. If it isn't, it's easy enough to do the granular version. Clearing all state should be preferred I think unless the optimization is needed.