Add "karma:unit:start" to "grunt serve" task

If you run the ```grunt serve``` task and then edit a file, this error pops up:

    There is no server listening on port 8080
    Warning: Task "karma:unit:run" failed. Use --force to continue.

    Aborted due to warnings.


Adding ```karma:unit:start``` to the ```grunt serve``` task solves this issue (every time i edit a file, tests are executed correctly).

P.S.: thanks for your generator, very nice job!