Add useStdErr option to call mstest with /usestderr

I've added an option to node-mstest to call mstest with the /usestderr argument. This causes mstest to report its errors (including failing tests but also failure to run the tests due to the resultsFile already existing) to stderr instead of stdout. 

This allows one to use the error callback of runTests to e.g., fail the build, and makes it easier to capture and display error messages. 

Please review. Thanks!
Actually, it might be useful for users if runTests() returned the child process instead of itself. Then users could pipe stdout and stderr themselves, and have the ability to kill it (e.g., if some other concurrent task invalidates the build). Thoughts?

(This is my first pull request, so as an aside: thanks for your response and consideration, and also for this project itself.)