add compress kwarg to both put and get methods

This patch simply adds a `compress` kwarg to both put and get methods that adds the `-C` option to the remote scp command. Originally contributed by @ebrevdo via jtriley/StarCluster#294

Sorry but I forgot about this one. It was submitted as a pull request to StarCluster's internal copy of scp.py when it should be committed here instead. Any chance to make yet another release? Should be the last one for a while (I hope :P)