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!
Wow, what was I thinking? You're right, it's better to be consistent and I haven't been able to think of a scenario where they need the buffer. I'll merge in your changes, update the changes and push it out with you attached as a contributor. Thanks again!

I'm not sure about passing back the child process but maybe having a `close` or `closeProcess` function would be handy. Because if they need the actual process then there's almost no point in using a separate module, in my opinion.