PTFM-5960: Initial GTable bindings

This is a very initial pass at making GTable bindings and is not expected to be particularly performant yet.  We may want to fix up a few things before we merge it in before the release tomorrow.  In particular, is there a more R-like way of doing what getRows does right now?  (It returns a data frame from a single API call and does not iterate to build the entire data frame.  Also, perhaps there is a more R-like name for the function that it does, e.g. "read" perhaps?)  Any other feedback is also extremely welcome.