Add Collision system for handling custom events upon collisions.

squashed commits:
Implementation of custom collision events. Some questionable design choices within. (Untested)

Updated collision system to new structure. Now uses CollisionHandlerComponent and CollisionSystem. Manifolds used instead of global callback.

Fixed previous compile error - a new one has taken it's place (uintptr_tr not defined)

Basic working state achieved. m_collisionCallbackKey assignment from lua does not seem to take effect.

Also storage and load still needs implementing, and agentabsorber system needs refactoring.

Storage and load implemented.
AgentAbsorber system & component refactored to use this system.
Acceptance tests performed

Collision filters added, not fully functional

Collision functionality nearly updated.
Needs cleanup, testing and integration with agentabsorber.

Working CollisionFilter class.

Attempt at exposing CollisionFilter to lua. Not successful.

Fixes to improve usage of CollisionFilter. Now uses Collision struct instead of typedef to pair of entityIds.
Better model for exposure to lua (still doesn't quite work).
Cleaned up documentation and header includes.

Fixed minor compile error

Collision system finalized.
- Added a member to Collision to represent the duration of collision represented since last clearCollisions().
  This member will allow more fine grained control of collision handling (damage every 0.5 second, etc).
- Cleaned up documentation and debugging code.

Renamed CollisionHandlerComponent to CollisionComponent.
Added removeCollisionGroup to CollisionComponent.
Added constructor taking a collision group to CollisionComponent

Refactoring of CollisionSystem. Stopped using mutable keyword in collision class and instead uses unordered map.
Added a vector<collision*> mirroring the contents of the unordered_map to allow iteration in lua.

Changed to using boost::map_values for iteration.

Rebased onto master.
Fixed setup.lua
Various relevant fixes.
Code is currently not functioning due to strange bug.

Implementation of custom collision events. Some questionable design choices within. (Untested)

Updated collision system to new structure. Now uses CollisionHandlerComponent and CollisionSystem. Manifolds used instead of global callback.

Fixed previous compile error - a new one has taken it's place (uintptr_tr not defined)

Basic working state achieved. m_collisionCallbackKey assignment from lua does not seem to take effect.

Also storage and load still needs implementing, and agentabsorber system needs refactoring.

Fix typo in Lua script

The previous version crashed because the game state passed to
CollisionFilter's constructor was nil in Lua and consequently
nullptr in C++.

Simplify GameState::findSystem

No need for complicated iteration. Also, dynamic_cast can only
throw an exception if a reference type is cast, not for pointer
types.

Refactor signature handling of CollisionFilter

Returning a pair of references is a little tricky because it's
not obvious how the std::pair template will handle it. Returning
a reference to a pair, however, is more predictable.

Use static initializers when possible

Minor refactoring

* Use typedefs (i.e. CollisionFilter::Signature) instead of repeating
the type
* Use emplace when possible, it's more readable than constructing
  the collection item manually
* Rename permutation -> combination

Do not register collision filter in Implementation constructor

At that point, the m_impl field of the CollisionFilter is not
initialized yet (because the Implementation object is not
fully constructed). The CollisionSystem asks the filter for
its signature, which is saved in the m_impl object. Since this
doesn't point to anything useful yet, it crashes.

Added CollisionFilter.init and CollisionFilter.shutdown to allow users to create their CollisionFilters in constructors.
Removed debugging code.

Minor refactorings for CollisionFilter and CollisionSystem

Removed typedefs in collision_filter.cpp.

Removed minor error in rebasing