[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.
If you move the assertion of the XML Structure and content to a private method then you can reduce the duplication between this and the next test and make your test more readable. I generally create a method called something similar to `thenTheXmlReportShouldContain` (with the variables as parameters). This is a syntax that matched cucumber/gherkin and makes tests easier to read.