Fixed Unbound Variable `edge` access when BinGrouper is empty
Incorrectly accessed variable `edge` instead of pre-filled value `start`.

Signed-off-by: Kevin Stone kevinastone@gmail.com

This bug surfaced when I was using a grouper on an empty array.  Since the for loop ends without assign edge, it leads to the UnboundVariableError.
 can post an example that reproduces (or even better, add a test for this error?)
and provide a release notes mention. thanks!
 Ask and you shall receive.
 can u setup Travis as well, see CONTRIBUTING

rebase into one commit and then push to have Travis run the tests

let me know if u need any help

thxs
 Lock and load.
 I can't take it with these RELEASE.rst conflicts. Merged manually via 99137af.

Thanks!
