Convert qrcode module to a service provider

  * Define an 'qrcode' provider contract
  * Use endroid/qrcode package for qr generation library, add to composer.json
  * Implement in Codex module

Eventually, this extension should be moved out of core, and into a composer installable module repository so that the library is only added if needed.
Remove obsolete preload file
Merge pull request #168 from geekwright/qrcode

Convert qrcode module to a service provider