Allow setting QoS parameters like prefetch size and count

Setting `qos_prefetch_size` will define maximum amount of content (measured in octets) that the server will deliver
(0 if unlimited - default).

Setting `qos_prefetch_count` will define maximum number of messages that the server will deliver (0 if unlimited).
Default to `bulk_size*2`.

Closes #38.
Closes #55.

(cherry picked from commit 90abc51)
Update to elasticsearch 1.3.0

Closes #61.

(cherry picked from commit ad1c3de)
Update to elasticsearch 1.4.0

Still need to fix Groovy scripts. See #71.

Closes #62.

(cherry picked from commit fa8466c)