camera rotation follow cursor

ping @ericsobral @vinioliveira

A raw version (not using SLERP) to make the camera follow the cursor. Here is the proof:

![screen shot 2014-06-14 at 8 09 56 pm](https://cloud.githubusercontent.com/assets/408219/3279848/6c94264c-f421-11e3-942e-243dd361dcd7.png)

This implementation is not suuuuper smooth. And maybe too fast. It's hard to have complete control of the rotation. Maybe I should look for how to do this using [SLERP](http://en.wikipedia.org/wiki/Slerp).

Another thing: I added a `tick` method to the camera. All game objects will have to have that method and will be called every frame update (so it can update itself). 

Currently I'm passing the `engine` to the object butI should pass different object that would have all data needed (like camera, buttons pressed, environment data, etc) for all game objects. I'll change this once I need it.

I decided not to make mouse movements events. Maybe I'll not make any input as an event. For now, I decided to check for inputs state during each frame. I think it make things less crazy and each object can do it's own check. (ex: check if the `jump-button` was pressed)

The event system will still exist but not for inputs. I think.

Thoughts?