Allow specifying pullup or pulldown when opening a pin

gpio-admin allows appending a "pullup" or "pulldown" argument when exporting a pin to set the Pi's internal pullup resistors. This change gives access to that functionality.

I wanted to avoid breaking API backwards compatibility, so all of the following will work:

gpio.export(16, "input", callback);
gpio.export(16, {direction: "input"}, callback);
gpio.export(16, "input pullup", callback);
gpio.export(16, {direction: "input", pull: "pullup"}, callback);

I haven't written tests for the change yet, but I plan on doing so as soon as I get my Pi hooked up again.