Implement basic BeingDefinitionAssets

Summary: Fixes T115. Implemented basic BeingDefinitionAssets. This implementation allows basic enemies. Energy is always out of 100 with a set regen? Though the specifics of that system do not matter until latter.

Test Plan: Review Code.

Reviewers: jrhodes

Reviewed By: jrhodes

CC: buildbot

Maniphest Tasks: T115

Differential Revision: http://code.redpointsoftware.com.au/D121