Added pointer-events on checkboxes and radios

[getninjaN]

Makes it work with FastClick on mobile devices.

[cvrebert]

@mdo Ping.

[mcrowe]

I also had trouble getting wtf-forms to work with FastClick. This fix didn't work for me though. Instead I added the following to my stylesheet (suggested here https://github.com/ftlabs/fastclick/issues/60).

label > * { pointer-events: none; }