Sequence events
Merge pull request #1497 from umpirsky/feature/sequence-events

Sequence events
Start porting Behat 2 into new & shinny Behat 3
Split contexts into smaller parts, more aligned with bundles
Update Behat feature files for easier testing & to prevent some issue
Update `composer.json` & `composer.lock` files to latest Behat v3
Update `README.md`, `.travis.yml` & `behat.yml.dist` files
Clean up a bit test environment
Clean database after, not before scenario
Polish Travis-CI & Behat configurations