Fixes #156: "Disabled radio button labels do not have the disabled class"

Minor tweaks so that the labels of radio buttons show as disabled, per the bootstrap docs.
* All tests pass.
* New tests for the changes are included (but feel brittle, I'm new to this and would suggest a second look by someone with more testing experience).

** This does not change check box handling in any way. I assume they have the same issue, but don't have any in my project to test with.