add WebP support

> WebP is a new image format that provides lossless and lossy compression for images on the web. WebP lossless images are 28% smaller in size compared to PNGs. WebP lossy images are 25-34% smaller in size compared to JPEG images at equivalent SSIM index. WebP supports lossless transparency (also known as alpha channel) with just 22% additional bytes. WebP also supports features such as animation, ICC color profile, XMP meta-data and tiling.

> https://developers.google.com/speed/standards/


### ENCODE IMAGE TO WebP

```
cwebp hello.png -o hello.webp
```


### LOAD WebP IN COCOS2D-X

```
CCSprite *sprite = CCSprite::create("hello.webp");
CCTextureCache::sharedTextureCache()->addImage("sheet.webp");
```

FOR RGB565, PLEASE USE PNG.


THANKS:

KunLun, Hui Wang