Added support for switching between test runners and auto-detect.

Added support for NUnit.
Merge branch 'master' of https://github.com/bookman25/vs-code-coverage into supportMultipleTestRunners

Conflicts:
	CodeCoverage/Scanners/TestScanner.cs
Fixed possible NPE
Forgot to remove one part.
Merge pull request #17 from bookman25/supportMultipleTestRunners

Support multiple test runners