move FieldCollectionItemEntity to a separate include file

In case the Drupal autoload registry gets corrupted, the field_collection module complicates rebuilding the registry because:
* it defines the FieldCollectionItemEntity class in its module file
* this class extends the Entity class
* the Entity class can not be found yet when the module file gets included while rebuilding the registry

To solve this, the FieldCollectionItemEntity should be moved to its own include file, this file needs to be mentioned in files[] in the module's .info file and then it gets dynamically loaded by Drupals autoloader.