Allow alternate variable names in import/export transforms

Resolves #143 and updated version and Readme accordingly.

I also added an "unknown" transform which will output an error message given any `///ts:???` where `???` is not a valid transform. There was a comment in CHANGELOG about adding such a feature and I figured I was already in there so I might as well do it.
This is why I had that hack http://typescript.codeplex.com/workitem/2337