add "speed" to CCBAnimationManager, and relative test code

# Purpose
In many situations, we want to change the speed of animations. For example, we may want to have differenct game characters with different walk speeds.

So I added  `_speed` property, which defaults to `1.0f` to CCBAnimationManager. Just call `setSpeed()` before running the animations.

# TestCase
I just reused the testcase of `ExtensionTest/CocosBuilderTest/AnimationsTest`，click any button repeatly would change the speed among [0.25, 0.5, 1.0, 2.0, 4.0]

I've tested with cocos2d-x-2.1.4 on win32,iOS and android
coco2d-x-3.0alphapre is only tested on win32, but it should be platform independent.