Task 7.9 - Add health to entities complete

Added max_health and curr_health attributes to the PhysicalState class
to track an entity's health.
Task 7.9 - Added tests to account for health.
Swapped the health args in the Physical State

Maximum health is less likely to be updated (i.e. more often to default
to '0') in the application of deltas. Therefore putting it as the last
argument seems to be a convenient way to avoid having to prefix the
parameters when creating the state deltas.
Merge pull request #39 from NintenJoe/Task-7.9

Task 7.9 - Added the health infrastructure for entities.