feat: add prefix config option

This patch allows users to prefix the title of growl notifications with a custom string. This is helpful in situations where you have multiple instances of karma running using the auto-watch feature.

Example Setup
----------------------
**karma-unit.conf**
```javascript
module.exports = function(config){
    config.set({
        //all your usual config stuff - files, excludes, etc.
        reporters: ['progress','growl'],
        growlReporter:{
            prefix:'UNIT TESTS-'
        }
   });
};
```
**karma-production.conf**
```javascript
module.exports = function(config){
    config.set({
        //all your usual config stuff - files, excludes, etc.
        reporters: ['progress','growl'],
        growlReporter:{
            prefix:'PRODUCTION TESTS-'
        }
   });
};
```

![screen shot 2013-08-24 at 2 37 18 pm](https://f.cloud.github.com/assets/4082216/1021228/e59eae5c-0ced-11e3-8e2f-c726911368f5.png)