Fixed 'pants goal targets'

As Jin pointed out in https://github.com/pantsbuild/pants/issues/333, pants goal targets is broken.  Its pretty easy to patch it up, but it relies on a fixed list of alias-> target classes, which is ugly and seemingly redundant.

Larry says we could just remove it. Or we could try to keep it going.  I'm fine with either decision, I just don't like broken commands sitting around, I think it sets bad precedent.