Specflow Execution Report - Changes to MsTestToNunit.xslt file to add "Tags" as Categories in the NUnit Report

Hi ,

   The Feature file of Specflow supports "Tags" (e.g. @SIT, @SUT or @JiraNumber etc) where the developers can tag their scenarios/features as required. When we use Specflow.exe with mstestexecutionreport command this information is lost while transforming the .trx file to HTML output. 

Therefore I've added a XSLT code for the same which would add a node as follows under "<nunit:test-case>" node 
 
![image](https://cloud.githubusercontent.com/assets/7948159/3351021/41b8e980-f9ee-11e3-8a38-3197c3216fbc.png)

   The users can thus take advantage of the tags and transform the output as per their needs. Further this would help the developers and stakeholders get a quick view of their test categories.