Add support for entitycache module

Entity cache only supports core entities. However, for contrib, implementing entitycache support doesn't seem too hard.

1) Adding this code to the controller object looks like it should work for tying into the caching system:

  public function resetCache(array $ids = NULL) {
    if (module_exists('entitycache')) {
      EntityCacheControllerHelper::resetEntityCache($this, $ids);
    }
    parent::resetCache($ids);
  }

  public function load($ids = array(), $conditions = array()) {
    if (module_exists('entitycache')) {
      return EntityCacheControllerHelper::entityCacheLoad($this, $ids, $conditions);
    }
    else {
      return parent::load($this, $ids, $conditions);
    }
  }

2) We have to add the cache bin to hook_schema() and a hook_update to create the cache bin. We should create the table even if entity cache isn't enabled, even though the table isn't used, otherwise management will be a pain in the keester.

3) We need to implement hook_flush_caches to add the cache bin.

4) There might be more I haven't seen in a quick examination of the module.