added radio support

this one is a bit more controversial. the annoying part is that `radio` groups don't have a parent html element they live under like `select`s do. jquery has a pretty nasty api for checking a radio groups value: http://stackoverflow.com/questions/596351/how-can-i-get-which-radio-is-selected-via-jquery

the first thing i did was make single `radio` elements behave like checkboxes. that seems pretty innocuous.

the second thing was that i allowed passing in an array (or NodeList) of radio elements, in which case it will treat them as a group, and return the value of the `checked` one. things got a little hairy, so if you see any ocd-ness or otherwise you want fixed let me know, happy to clean anything.