fixed files with parentheses in their names bug

I tried to create sprites files for a bunch of png's with no success, so I started debugging `tasks/montage.js` and found the error could occur if any file contained parentheses in it's name.

E.g. `montage -tile 17x -geometry 80x80  assets/abacus-(2).png` would fail, so you have to either escape file names or enclose file name in quotes. In my pull request I use quotes.