Whitelisting various input types

To play friendly with text input fields, Ribs currently ignores any hotkeys when the selected element is an `<input>` element.  However, this prevents hotkeys from working on non-text `<input>` fields, such as radio buttons or checkboxes.  This pull request changes that.

While I was not able to find a clever way to filter supported elements without white/blacklisting them, it looks like `type='radio'`, `type='checkbox'`, and `type='button'` should resolve most occurrences of this issue.

Here's a chart of which elements are text entry fields, http://www.whatwg.org/specs/web-apps/current-work/multipage/the-input-element.html