Finer grained control over when offsets are committed to zookeeper.

Committing offsets adds load to zookeeper. Since zookeeper clusters
cannot be scaled horizontally, clients may opt to to reduce the load on
zookeeper by relying on automatic commits (via auto.commit.enable and
auto.commit.interval.ms consumer properties) rather than having the
offsets committed after every batch of events are processed.
Fix broken logic in getting value for committed offsets, add unit test.
Added more detail to javadoc.
Bump the version.
Merge branch 'master' into optionally-disable-per-chunk-commit

Conflicts:
	pom.xml
Merge pull request #1 from jrothfeder/optionally-disable-per-chunk-commit

Finer grained control over when offsets are committed to zookeeper.