[TestFest] Added unit tests for Writer\Statistics

![](http://d3img3do1wj44f.cloudfront.net/wp-content/uploads/2013/10/image1-432x240.jpg)

Added unit tests for the phpDocumentor\Plugin\Core\Transformer\Writer\Statistics class. Getting 100% coverage now :bowtie: 
Also fixed a typo and added some more verbose documentation about and $element in the class itself.
Tip: Avoid using the `@covers` annotation on a test class. I have done that in the past and have been bitten multiple times because I thought code was covered by a test while it actually was not.