EhCachePlugin refactoring

I want to monitor EhCache using Metrics & Graphite.
Metrics already have a special decorator for EhCache. 
But unfortunately current design of EhCachePlugin doesn't allow to use it. 
Also I have no ability to reuse EhCachePlugin at all.

I propose to change cache's type to more generalized net.sf.ehcache.Ehcache istead of net.sf.ehcache.Cache and use getEhcache() method, which returns decorated caches. Also it would be good to factor out CacheAPI implementation for EhCache.

https://groups.google.com/forum/?hl=ru&fromgroups=#!topic/play-framework-dev/n3vJ8Q579S0