Add a binding based on ZooKeeper.

Although this implementation has severe performance problems, as
it makes a write-intensive use of ZooKeeper, it is is easy to
setup and provides standard non-partitionable VSC. It originally
was avaiable with a custom interface at:

	https://gitorious.org/groupz
Minor changes to Groupz code to better fit JGCS.
Implementation of the standard API on ZooKeeper (aka groupz).