Added "none" as a label position option and now respects the checkbox's disabled state

I've made a couple useful changes to hold the project over until more formal features have been implemented.

One change is to respect the original checkbox's disabled state. I added a new class to give the checkbox a disabled look by using opacity and made the link unclickable. I think the better approach to the disabled look would be extending the checkbox image to include disabled versions, but I'm not good enough with photoshop to update the psd.

The other addition was to make a label position of none so that a label isn't displayed when it's not required.

I've never done a pull request before, so I hope this works.