Version 0.12.0

More sweeping changes.  Should be considered incompatible with versions earlier than 0.11.0

Added a bunch of utility functions to base entity
LeveledXMItemEntity is now a generator
Added FlipCardXMItemEntity generator
Added basic inventory items and sensible defaults via the new generators, instead of in their own files
Updating asset manager to remove old references to models
Adding ability to preload a single entity via the asset manager