Added Velocity.FileCopier

I open this pull-request instead of committing directly, so you can give me feedback and @rissem can try it out. (https://github.com/xolvio/velocity/issues/51)

Velocity.FileCopier is a worker that can reactively copy test files to the app space of the mirror.

It assumes this structure:
* mocha-web
  * client
    * mySpec1.js
    * mySpec2.js
  * server
    * mySpec1.js
    * mySpec2.js

I tested it with my jasmine package and the velocity-example.

@rissem Can you try to integrate it into mocha-web and give me feedback if something doesn't work for your case.