Add FreeTrace.hs

Following up on the discussion in #9. It compiles but I haven't tested it.
Well, maybe not so easy after all. See [Gabriel's comment](http://stackoverflow.com/questions/17511841/monadtranscontrol-instance-for-proxyfast-proxycorrect/17515535#17515535) and [this one](http://stackoverflow.com/questions/23744439/a-monadtranscontrol-instance-for-freet).