Revisit initialization

[rmosolgo]

There are a lot of issues popping up regarding how react-rails sets up & handles app initialization. I want to work out these problems.

To do

• Ensure support for .js.jsx.erb (Can't use view helpers #231)
• Fix jsx_transform_options when precompile doesn't initialize (assets:precompile broken with initialize_on_precompile=-true #192)
• Fix long boot times (Triggering Sprockets on Rails init #249)
• Ensure support for Sprockets 3 (Sprockets 3.0 support #240)
• Fix file copy permission issues (initializer react_rails.setup_vendor fails if gem source files were read-only #175)
• Address sprockets-rails compatibility issue (fix(prerender) accept string props #268 (comment))

[rmosolgo]

I have some ideas to kick things off:

• Instead of copying files into the app directory, add different gem directories to the asset pipeline based on config.react.variant. (For example, if :development, then add /vendor/development/, whose copy of react.js is the dev build. If :production, add /vendor/production, whose react.js file is the production build. That way, no file copying on boot.)
• Decouple the server renderer, as suggested here: Better support for Flux architecture #143 (comment)
• Write some failing tests for the initializers, fiddle with em til they work!

[rmosolgo]

.jsx_transform_options is a wont-fix for me, the work around is to assign JSX.transform_options directly and I think that's fine