Refactor the Issue Tracker forms in App
Simplify AppsController

 * avoid a filter to parse IssueTracker params
 * Add acceptance test with phantomjs to be sure the Apps editing works
   fine with IssueTracker
 * Add IssueTrackers::None the IssueTracker configure doing nothing
Add information in developper test about skipping js test
Extract all IssueTracker system and migrate to errbit_plugin

This new system can avoid having all dependencies in Errbit directly.
Now if you want new issue tracker you need add it. Not delete it like
previous.

This work is in progress actually
Update Gemfile with errbit_plugin and errbit_github_plugin
Update errbit_plugin to have all assets and all test pass