Dynamic support for Cache Providers using JCache

Gora should be able to support external Cache Providers, such as Ehcache. MemStore would be the first candidate to use the Cache Provider but ideally caching should be independent from the back-end datastore module. So, users can combine the back-end datastore of their choice with the Caching Provider of their choice. The caching module should be dynamic in the sense that different Cache Providers may be supported.

Benefits for Gora:
1) Caching may provide significant performance improvements
2) Integrate with Ehcache; Java's most widely-used Cache (fast, simple to use, small foot print)

Ehcache is under the Apache 2.0 license.