Add support for remove_watches operation

[hannosch]

ZK 3.5 adds a new remove_watches operation, implemented in the C library in https://issues.apache.org/jira/browse/ZOOKEEPER-1887 and parent issue https://issues.apache.org/jira/browse/ZOOKEEPER-442.

This is the new type code 17.

[hannosch]

Looks like this might already exist in a fork at rgs1@44ca48e

[hannosch]

Actually this is covered by PR #189.

[rgs1]

i'll rebase my branch on master + add some tests

[StephenSorriaux]

Partly done in #715