Cekit

* Added support for module versioning - http://concreate.readthedocs.io/en/latest/descriptor.html#module-installation
* Added proper overrides documentation
* Fixed overrides support for modules (92)
* Improved OSBS shell session - Concreate now prefix PS1 variable to show you are in a Concreate session (84 )
* Fixed fetching URL resources, if url is specified and cacher is missed, it will try original url (77)
* Fixed default values handling for run and execute
* Deprecated --build-tag option, you should use --tag instead
* Fixed possibility to have duplicity environment variables in generated Dockerfile (119)
* Fixed rhpkg new-sources causing git to ignore some file changes (123)

* Exit with proper exit code when tests fail to execute
* Updated path to repository with steps

* Added support for tests. More information can be found on dedicated documentation page about testing: http://concreate.readthedocs.io/en/latest/test.html
* Updated documentation to make it easier to understand
* Added missing support for defining volumes.

Fixed issue where generation failed when the tool was executed in a different directory compared to where the image descriptor is stored.

All Python 3 incompatibilities with decoding strings should be now fixed.

Another fix for Python 3 incompatibility with decoding string.