[GEOS-6790] Add support for GWC In-Memory caching for TileObjects

This pull request contains the fix for JIRA: http://jira.codehaus.org/browse/GEOS-6790. Note that this pull request requires to merge the related GWC counterpart (https://github.com/GeoWebCache/geowebcache/pull/295). More info about the GWC improvements can be found at the following link: https://github.com/GeoWebCache/geowebcache/issues/294.