Fixed EntityWorld.CurrentState

When getting EntityWorld.CurrentState NullReferenceException was thrown
when iterating this.EntityManager.ActiveEntities with holes (after
deleting any Entity).
Fixed by null check.
Fix Aspect Exclude to handle multiple types correctly
Merge pull request #73 from tonedev/aspect-exclude-fix

Fix Aspect Exclude to handle multiple types correctly
Added unit tests for Aspect

- Exclude tests
More unit tests for Aspect

- Aspect.Empty, .All, .One, .Exclude
- clarified Aspect.Empty meaning
Added Aspect unit tests call to Program.cs
Included TestAspect.cs to mono unit tests
Merge pull request #74 from Maximusya/aspect-unit-tests

Aspect unit tests
Merge pull request #8 from thelinuxlich/master

Sync with master repo