[WIP] React v0.13

[sollycatprint]

From @zetachang on April 1, 2015 12:49

Tracking progress of upgrading to React v0.13

• Integrate master's React::Element work
• React::Testing added to clean up testing code
• native_alias all the life cycle methods
• Make react-source as runtime dependency
• Fix all the test warning
• Separate code run in Ruby in opal namespace
• Clarify the dependency with React.js (only depends on core rather than add ons)
• update README
• drop sprockets 2.0 (as opal master is dropping it)
• ReactComponent access from this.refs is not ruby object
• update examples
• present(Foo).on(:event_name) won't work
• raise error when deprecated method invoked
• Object.assign shim

Copied from original issue: zetachang/react.rb/pull/17