Refactoring of tiles.py

[Ziexe Ujhijr]

Hello. 

When i work on my game, i try to use A*, and in a moment start to refactor tiles.py :)

First of all, i merge changes of  https://github.com/los-cocos/cocos/pull/304, so my refactoring based on this.

Commonly, i split code of `load_tmx` to "screen-fitting" size function and this refactor has fix some issues with loading.

For example, with next map(created with Tiled), that has separated tileset file can't be loaded. It's crash with "UnboundLocalError: local variable 'tileset' referenced before assignment".

I attach all that needed to reproduce this:

1. put *.png, *.tsx, *.tmx in directory assets in test-root

2. put testing.py in test-root

3. create env, install cocos2d(0.6.5+)

4. run "python testing.py"

In my case, it's files on Koraken branch "tiles-301", and origin versions "0.6.6", "0.6.5". 

I'm check utests, but no changes. 18 fails as was before my changes. Looks like there is no tests for loading tiled maps :). I'm going to write tests in future.

So , changes lie in my fork, i create PR from dev to master for easy review. I will be very grateful for the review. 

Best regards.