Typecast $lifetime to avoid issues when false in Cache_Database::set_with_tags().
Revise how ORM_MPTT finds the model name
- move from __construct() to _initialize(), similar to ORM
- change $model_name --> $_model_name since it's private
- use class name instead of table name, similar to $_object_name.  This maintains
its capitalization (e.g. "Model_G2Map" --> "G2Map", not "g2map" or "g2_map")
Add ORM::merge_order_by() since order_by() no longer accept arrays.
Change "->find_all($limit)" --> "->limit($limit)->find_all()".  Also, in
Hook_ImageBlockBlock, change "->find_all(1)->current()" to just "->find()"
Change "->find_all($limit, $offset)" --> "->limit($limit)->offset($offset)->find_all()".
In ORM_MPTT, call find_all() only after constructing a new ORM model.
Merge pull request #281 from shadlaws/more_db_orm_fixes

More db orm fixes