Update README.rst

Fixed some formatting and clarified some instructions. I was editing this on the master branch before, but I figured I might want to ask permission instead of running around like I own the place.
I wouldn't include ``bash`` in the Unix/Mac instructions, as I don't think that works in all cases...   that said, perhaps ``./javaviz-server`` might be a good idea (it should be already marked as an executable, so that should be sufficient).