Moved offset initialization to get_offsets method.

The ability to pull offsets defaulting at 0 is helpful in initialization for
auto commit but is also useful code for times when auto commit is not turned
on, so it is nice to have as a method.
I think this internal callback function should be dropped in favor of explicit error checking of the response after calling `send_offset_fetch_request` .  If we try to get offsets for an unknown partition, shouldn't we raise an exception here?  Usually that signals our metadata is messed up and a full metadata refresh is in order.