Added decoder for RGBA images

additional changes:
* Consolidated the encode wrapper functions in webp.c
* Added wrappers for WebPDecodeRGBA and WebPGetFeatures to webp.c
* Removed unused variable quality_factor from WebPDecodeRGB_wrapper in webp.c
* Changed _accept in WebPImagePlugin.py to match the documented WebP header (http://goo.gl/dBVdF)
* Made _save in WebPImagePlugin.py raise IOError if mode isn't RGB or RGBA