Adding hbase-index contrib/ project - support #4

For the moment, pushing this out as an independent, contrib project. Its independently build-able from Phoenix core until we can figure out where it should go and if it actually works all the way.

To be fair, this is actually the product of extensive discussions at Salesforce.com. A full description of the underlying architecture is available here: http://jyates.github.io/2013/06/11/hbase-consistent-secondary-indexing.html, but a small outline and usage can be found in the README. 

This should provide the framework for building an index maintenance tool - specificaly a PhoenixIndexBuilder - that can support all the phoenix semantics in a consistent manner.