implemented treat warning as errors commandline option.

Same name as csc.exe uses: [msdn](http://msdn.microsoft.com/en-us/library/406xhdz3.aspx)
Issue #828

Feedback appreciated but feel free to close when you don't want this functionality at this point :)
I believe it did, runBaseline calls generateActual: https://github.com/Microsoft/TypeScript/blob/5ce3baf339e0424263069c70f0ea239bc3cea96a/src/harness/harness.ts#L1307 which creates the file when it is something different than null. It throws an error when it is undefined.