Remove JSX support

[zetachang]

After discussing with @catmando, to support JSX, extra dependency on sprockets make it hard to maintain backward compatibility either for opal 0.7 or rails 3.

And given that the integration with react-rails gem is possible, we could simply rely on JSX compiling feature provided by it rather than rolling our own.

[sollycatprint]

This issue was moved to reactrb/reactrb#46