Added missing xdg-open dependencies.

xdg-open calls xdg-mime in two places ([line 584](https://github.com/pwnall/node-open/blob/master/vendor/xdg-open#L584) and [line 601](https://github.com/pwnall/node-open/blob/master/vendor/xdg-open#L601)), but the xdg-mime dependency is missing. This pull request adds xdg-mime and the utils file it depends on.

The only tricky part is that [there is no reliable way to reference another shell script](http://mywiki.wooledge.org/BashFAQ/028). So, I changed the JavaScript a bit so the location of the shell script is passed in as an environment variable.

I came across this problem while trying to run [grunt-contrib-watch](https://github.com/gruntjs/grunt-contrib-watch) on [Nitrous.io](https://www.nitrous.io/join/d41_7dXKTp0).