Adds Entity caching support
Removes old caching system for property datatype
Adds TODO for tests that calls the API
Adds WikibaseDataModel version to cache keys

Avoid the need to purge cache when the library is upgraded
Use Memcached on localhost as default (and ArrayCache as fallback)

Note: pretty bad. We should write a proper configuration system.
Merge pull request #26 from ProjetPP/cache

Implement caching of entities