reorganize library.py
move `remove` method to model objects

--HG--
extra : amend_source : 31d3ccbdde816b82bb5ec943ecb870e0df699dd0
move `move` method to model objects
eliminate lib argument to `Item.evaluate_template`
move `Destination` method to Item class
remove magic strings from Library.__init___

We now pull the table names from the models.