Modules

This pull request adds module functionality which covers #289. The DraggableEntity and DroptargetEntity are refactored to modules and so is the implementation code (D&D & multitouch examples and functional tests).

The modules use the module pattern. For convenience there is also a 'me.import' method added, which can optionally replace the functionality of the self executing function around modules (used in the examples).

The 'mix' method can be used to be able to mix in all needed abilities for a module. This functionality also paves the way for #211, where we can separate out the platform game specific logic into a new module, which then can be 'mixed into' the base entity.