allow template strings for attributes in jsx

[monsanto]

It'd be cool if <div className=myClass ${class1} ${class2} /> worked, instead of having to wrap the template string with {}.

[zpao]

Can you propose this in https://github.com/facebook/jsx/? The backtick is not a part of the spec we wrote there so we can't even implement it until it's there. Let's revisit this then.

[monsanto]

done facebook/jsx#25