[Issue #514] Allow result iteration in GroupBy ResultTransformer

Here's a pull request for Issue #514 consisting of iteration implementation itself and a test for it. I added access point to GroupBy although it didn't fit in as nicely as I'd liked it because of the signature differences.

An example usage scenario would be against MySQL streaming mode which send rows one by one from client to server thus avoiding the time/space consuming temporary storage creation.

PS. One has to use order by to ensure that grouping can be done. Un-ordered iteration over [a,a,b,a,c] will yield to four groups instead of three which might not be advisable in most cases.