Modification for Windows compatibility

[athomic1]

Run according to documentation, was failing with the following error:

gifsicle.exe:/tmp/a.gif: No such file or directory

Located two references to /tmp/a.gif in gif-to-sprite.js (lines 31/32):

child_process.execSync(gifsicle + ' --colors=255 --output /tmp/a.gif ' + fileName);
var gifsicleCmd = util.format('%s --unoptimize --resize %s --output - /tmp/a.gif',

After replacing '/tmp' with '%temp%', program ran and successfully converted files.

While this "fix" allows the program to run in a Windows command shell, it most likely breaks it for other systems. Would suggest detecting OS and setting a tmp path variable accordingly, to be referenced in place of the hard-coded path in the lines above. If I get time, I may try this myself, but as I've never used node.js before, it's likely to take awhile. Someone else might have better luck hitting on a solution sooner.

[kristofferjalen]

+1 I also ran into this issue.