CFY-783 added cancel workflow support to workflows infrastructure and tasks_graph implementation
CFY-783 merge commit
CFY-783 changed api.is_cancelled name, added doc
CFY-783 - added documentation to tasks_graph; has_cancel_request() in workflows api returns False instead of None
CFY-783 added more comments to tasks_graph
CFY-783 added handling for when executions get cancelled before they're even started
CFY-783 modified travis.yml for tests
CFY-783 flake8 fix
CFY-783 reverted travis yml back
Merge pull request #15 from cloudify-cosmo/feature/CFY-783-add-support-for-cancel-execution

Feature/cfy 783 add support for cancel execution
Merge branch 'develop' into feature/CFY-799-task-retries