Sync out latest JSX transforms

The biggest change: instead of calling functions directly, we now use
React.createElement to wrap.

The other big change: this removes the need for the @jsx pragma.
Instead we'll parse everything, assume React is in scope,
and only look for the actual XJS parts in the AST (as opposed to only
runing the transform when @jsx was specified in the docblock).

Note: this is actually a series of commits internally and should have
been syned out in pieces over the past several weeks.