Added support for descriptions in scenarios and scenario outlines so that the parser does not raise an exception if they are encountered
Added description field and comments to Scenario class
Added missing state changes for scenarios and scenario outlines in the action_steps method
Merge branch 'master' of https://github.com/caphrim007/behave into caphrim007-master
Pull-Request #79: Provide Support for Scenario Descriptions
CHANGES:

  * Add description attribute also to ScenarioOutline ctor.
  * Move description parameter to end of Scenario/ScenarioOutline ctor.
    => Problem because parameter was already used by steps.
  * Extend JsonFormatter to store Scenario/ScenarioOutline description.
  * Extend json_parser to handle Scenario/ScenarioOutline description.
  * Cleanup/refactor parser to avoid duplicated code fragments:
    => _build_*_statement()
    => subaction_detect_next_scenario() to avoid duplicated code fragments
    => action_next_scenario (tags and more...)
    => Add check that Background does not support tags.
    => ParseError: Add stripped line to exception message (better context).