Implement feature flags for smoother roll outs

[webknjaz]

Some info on the idea of the Feature Flags and implementations:

• http://featureflags.io/python-feature-flags/
• https://github.com/disqus/gutter (UI: https://github.com/disqus/gutter-django)
• https://github.com/ashcrow/flagon
• https://github.com/brechin/pyrollout
• https://github.com/gregbaker/django-featureflags

[webknjaz]

Ref #63