commitOffsets can be passed the offsets to commit

This adds another version of `commitOffsets` that takes the offsets to commit as a parameter.

Without this change, getting correct user code is very hard.  Despite kafka's at-least-once guarantees, most user code doesn't actually have that guarantee, and is almost certainly wrong if doing batch processing.  Getting it right requires some very careful synchronization between all consumer threads, which is both:
1) painful to get right
2) slow b/c of the need to stop all workers during a commit.

This small change simplifies a lot of this.  This was discussed extensively on the user mailing list, on the thread "are kafka consumer apps guaranteed to see msgs at least once?"

You can also see an example implementation of a user api which makes use of this, to get proper at-least-once guarantees by *user* code, even for batches:
https://github.com/quantifind/kafka-utils/pull/1

I'm open to any suggestions on how to add unit tests for this.