Use change event

[raycohen]

This PR is an attempt to simplify the implementation and more completely cover the ways that a radio button can become checked, and emit less additional DOM elements when not used as a block.

There are significant changes:

• Non-block form of component now emits only an input[type=radio] element
• Block form emits a label element wrapping the input and content. No more span.
• checked and disabled are not bound as classNames (they were on the span previously, perhaps they should be on the label element)
• Added meaningful tests.

[toranb]

Wow this looks great! I really like the changes (greatly simplified this component for those reading the code).

[lukemelia]

Changes looks good to me. 👍 We may want to branch current master as pre-htmlbars or something.