Enable processing of many sprites

First of thanks for this awesome module :)

However i ran into two minor problems, in case u want to export a lot of sprites ( a couple of hundreds).

The first issue is that node will throw an error because too many files are open. This can be fixed by using 'graceful-fs'

The other issue is that it takes a lot of time to render all the differences onto the final spritesheet, this can be fixed by using imagemagicks 'convert' with the composite option instead of the composite tool itself. Convert is able to handle bulks of images at once which speeds up the process alot.

Hope the changes make sense.