M_PI isn't standard...

* Replaced reference to M_PI with holo::Math::Pi
Make handle index unsigned
Added world unit test

* Added world unit test
* Refactored holo::Entity::AddComponent to return the component handle
rather than a boolean
* Fixed bug in holo::World::World where the entity handle type was used
to access the component category, rather than the component handle type
(thanks unit test!)