[v2] Create slick.world.frame method to update entire world.

[erinmaus]

• Allow setting a velocity on entities and updating the entire world at once with a slick.world.frame method that takes a deltaTime. All entities will be simulated, given their velocity and the elapsed time, to prevent any potentially tunneling caused by one-by-one updates. Currently, update, etc treat all other entities besides the one being updated as being stationary.
• Add local collision filters and local collision response handlers to slick.entity:
  • The collision filters will be used during slick.world.frame to see what objects interact with this one. They can return visit functions as well like the current collision filter functions.
  • The local collision response handlers, if set, will be used in place of the global collision response handlers. The default behavior will be to use the global collision response handler.
• Add events like startContact, stopContact, move and potentially others to slick.entity. A function provided to this event will be called when the entity starts or stops touching another entity or shape, for example.

To keep it generic, all methods that take a function can take a table with a __call metamethod. And to keep it simple, only one function can be registered at a time to an event, collision filter, collision response handler, or other things that use functions provided externally. More advanced behavior will require use of pub-sub libraries or such!