feat(ExecJSRenderer) add ExecJSRenderer

[rmosolgo]

This ExecJSRenderer was created while testing server rendering performance and I thought it might be a chance to reorganize server rendering code. In this branch:

• ExecJSRenderer is basically a wrapper around ExecJS
• SprocketsRenderer is an ExecJSRenderer which fetches source JS from the asset pipeline & enables "console replay"

What do you think?

Pros:

• ExecJSRenderer was great for testing performance. I'm not sure what other use it could have but it might come in handy again.
• ExecJSRenderer's #before_render and #after_render hooks add extensibility as requested in Decouple renderer #253 (comment)
• It demonstrates how Renderers fit into react-rails, maybe people will make their own

Cons:

• The split is a bit arbitrary, specifically:
  • Where should "console replay" be implemented?
  • Which renderer decides between renderToString and renderToStaticMarkup ? Right now, that's very awkward.
• ExecJSRenderer might be a pointless exercise because react-rails depends on sprockets, why wouldn't you use SprocketsRenderer?

Anyways, curious what others think of this change!

[rmosolgo]

no news is good news