Dynamic webpack publicPath entry

[jhen0409]

Related to #40, https://github.com/jhen0409/react-chrome-extension-boilerplate/pull/37#issuecomment-217719393.

• The iframe of injectpage cannot get correct path, it need to get parent page protocol.
  • In development mode, the publicPath must be {protocol}://localhost:3000/js
  • In production mode, the publicPath must be chrome-extension://{id}/js
• Remove injectpage HMR, looks GitHub updated CSP.
  The shield icon also not appeared. (cc @zalmoxisus)