fix for transparent option

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