refactor .requires, documentation for .requires and .addComponent

Now that addComponent does not re-add components that are already added (see https://github.com/craftyjs/Crafty/pull/428), it has become exactly the same as requires. Therefore I deleted the redundant code and clarified the documentation.

This does not preclude the possibility that requires and addComponent could be re-differentiated in the future (some vague proposals were mentioned in discussion at https://github.com/craftyjs/Crafty/pull/428 ). That's a different issue. In the code-base as it exists today, the two functions are synonyms, and we might as well make it easier for people to understand that.