fix for transparent option

I can't successfully set transparent color so I dug into the code and found that in GIFEncoder.js index was used to query an array,but index was a floating point number calculated by i / 3. 

Besides when converting frames to tasks, transparent was omitted so the transparent color didn't pass to GIFEncoder.js.

so I added "parseInt(i / 3)" in GIFEncoder.js and  "transparent: frame.transparent" in gif.coffee getTask function and everything works now.