"checked" property in React.DOM.Props should be of type "Boolean -> Props"

[foopq]

I believe the "checked" property in React.DOM.Props should take a Boolean instead of a String. It feels more idiomatic to use true/false instead of [some string]/[empty string]. Both booleans and strings will work in React, it just feels like booleans are a more natural choice.

[ethul]

I agree it would be handy to have a function that uses true/false. Perhaps we could add a new function checkedBoolean to support this. It also might be good to run through the props to see if we could do this for other properties as well.

[ethul]

Resolved by #88