Ensure ZooKeeperMap is immutable.

The ZooKeeperMap implementation is expected to be immutable, however it
can be mutated by using Iterator.remove() on the returned entrySet(),
keySet(), and values() collections. This changes the delegate() to
return a Collections.immutableMap() instead of the current behavior
(overloading put/putAll/remove/clear).
Ensure that test runs to end.
Whitespace fixes.
Keep lines under 100 chars.
Merge pull request #144 from pkwarren/zkmap_immutable

Ensure ZooKeeperMap is immutable.