Fixed issue when using Memcached as the Play! cache implementation

[tazmaniax]

Greenscript was adding a value to Memcached before the value had been populated. Change was to wait until the value was populated and then add to Memcached.

1. Add protected onWriterClose() method to BufferResource that is called when the writer is closed. Defaults to doing nothing.
2. Updated GreenScriptPlugin.bufferLocator_.newBuffer(List
   resourceNames, String extension) to anonymously extend the
   BufferResource instance and define the onClose() method to do
   the Cache.set(...)
3. Removed unnecessary @OverRide annotations