nimbus ha solution for issue STORM-166

Nimbus HA feature is quite important for our application running on the storm cluster. So, we've been working on the problem for some time and now a solution seems not that perfect but be enough to apply has comed out.

1.Nimbus Servers now can register themselves in Zookeeper. They perform a leader election using "InterProcessMutex" interact with Zookeeper to ensure that there is only one nimbus responsible for launching and monitoring topologies.

2.Every Nimbus Server is running a timer to compare and find if there are topology codes which are not exists on it's local disk. They would download lcoal missing topology codes from the Nimbus leader through the thrift RPC just like Supervisors do.With this feature, any numbers of Nimbus Server can be launched through out  the cluster.

3.StormSubmitter,Supervisor,Non-leader Nimbus and Storm UI now are able to find and connect to the Nimbus leader via Zookeeper.A Nimbus leadership table is also added to Storm-UI on the main page to show every Nimbus's leader-election state and it's host in addition.

PS: Some implementation of the Nimbus-Election part has taken @Frostman's solution for reference(link: https://github.com/nathanmarz/storm/pull/422).