Move fetching last known offset logic to a stand alone function.

The `Consumer` class fetches the last known offsets in `__init__` if
`auto_commit` is enabled, but it would be nice to expose this behavior
for consumers that aren't using auto_commit. This doesn't change
existing behavior, just exposes the ability to easily fetch and set the
last known offsets. Once #162 or something similar lands this may no
longer be necessary, but it looks like that might take a while to make
it through.
also reset `self.fetch_offsets` in `fetch_last_known_offsets`
Merge pull request #177 from zever/fetch-last-known-offsets

Move fetching last known offset logic to a stand alone function.