Added rudimentary support for tile collisions.

The collision detection code is not elegant right now, need major
reworks and refactoring. The unnecessary comments also mess up the code.
Added world bounds collision for hero.

The implementation is same with tile collisions with minor modification.
Edited license.txt

Fixed some text problem.
Revert "Edited license.txt"

This reverts commit dca483b95fbaae4b56fb602a72f5ab110e6d25ce.
Version 0.0.1b released.

* New: Hero will collide with world bounds now.
* New: Added hero's animation support.
* New: Collide with tiles.
* New: Slowing the animation speed while colliding with tiles and world
bounds.
* New: Added constants.js to manage unchangeable codes.

* Updated: Changed the look of the stair in hero's house.

* Fixed: Removed collidable layer of hero's house in Tiled.