Add a `second_latest_offset` magic offset symbol to fetch all new events plus the one event which occurred most recently.
Add negative offset support

Note that this removes the ability to pass what a negative number means in the
Kafka protocol. This means a special Poseidon symbol must be passed instead.
Fix case when negative offset is past beginning of partition
Merge pull request #24 from Shopify/second_latest_offset

Negative offsets