Define SG (for snappy iovecs) in snappy.c, where it belongs.
Proper length validation for Kafka Strings and Bytes
Added "group.id" configuration property.
Added broker based offset commit store (issue #85)

 * Offsets can now be commited and read from the broker.
 * Requires Apache Kafka 0.8.1 on the broker
 * added config property offset.store.method = file|broker  (default: file)