Config & initialization fixes

[rmosolgo]

• Accept transformer class from config
  • Right now the only way to do that was assign React::JSX.transformer_class = which is ok with me, but I guess a Rails config is more normal
• Don't reload the transformer in dev
  • Make development match production
  • If you change your JSX config, you'll need to restart the app anyways
• Move JSXTransformerTest
  • one class, one file: works nice with Guard, makes it easy to find the tests for a class
  • (I know not all of react-rails is on that pattern yet but i'm working on it :)
• fix(ServerRendering) default pool_size of 1; document deadlock & ther…
  • based on findings from [testing] Execjs runtime benchmarks #290
• refactor(AssetVariant) extract asset build & path logic
  • just want to DRY it up and get logic out of the Railtie

[danott]

Great work! 💯