Added support for 'start' in background and 'run' goals

In our build infrastructure, we cannot start a browser from within the build process.

So I added the possibility 
- to start a karma server in the background (goal 'startServer'). The server keeps running so that browsers can be connected to it whenever needed.
- and then use this server to run the tests on all connected browsers (goal 'run').

What do you think?