Add `karma.stop` api method

In certain development cases there may be a large number of unit tests that take many seconds or minutes to run. In these cases the developer may be actively programming and saving, which could execute gulp watch events that force another unit test run. Currently when a set of unit tests is running Karma can execute another set asynchronously.

It would be nice if the developer could develop a method to stop a running task before starting a new one.

Example:

```
if( karmaIsRunning ){
karma.stopRun(); //stops actively running tasks before starting a new set of tests.
}
karma.start(config, function(){});
```