Add MoveSet, Nature

The MoveSet class will represent all of the Moves a Species can learn.

The Nature class will represent the different natures a Pokemon can
have. The Nature will primarily be used during combat for various
calculations.
Add convenience methods to Move; Add accessor of MoveSet to Species
Add Npc and NpcTrainer
Merge branch 'feature-npc' into develop

* feature-npc:
  Add Npc and NpcTrainer