Implement spread attributes

In relation to https://github.com/jsdf/coffee-react-transform/issues/19
Roughly as specified in https://github.com/facebook/react/commit/e6134c307e2bb7765aaa747eb5d2136fc18abbd7
(Moreover, if it were to be supported, it would probably mean `{...x, y: y}` and you would write `{...x, ...y}` for what you wanted but you can just write `{...x} {...y}` in this instance anyway.)