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!
My reason for this was that since the parser is emitting a string on the error event in line 121 above, it would be more consistent for it to do the same here. And that I didn't think a user would ever want to do anything else with the buffer than toString it anyway..

But I'm just as happy stringifying the response in my callback if you prefer.