Features:

• Added World::getEntities()
• Added World::clear()
• Added Movement example
• Added Collision example (thanks to johnmarinelli)

Bug Fixes

• Fixed Entity::Id's being re-cycled incorrectly.
• Fixed issue where re-activating Entities caused them to be re-added to their Systems.
• Added the ability for an empty Entity handle to check its validity without asserting.
• Removing Component caused crashes

Thank you jabza for helping to fix these bugs.