[WIP] BDD: Fix behat contexts inheritance

This belongs to a inheritance fix that involve:
- [x] ezsystems/BehatBundle#
- [x] ezsystems/ezpublish-community#154
- [x] ezsystems/ezpublish-kernel#872 (EzPublishLegacyBundle and EzPublishRestBundle)
- [x] ezsystems/DemoBundle#90

Todo:
- [ ] Make Mink only inherited by Browser
- [ ] Make an API context to be the main context for API's
- [ ] Update structure
- [ ] Update doc with sentences in BrowserContext 
- [ ] Move sentences interfaces to the right place
- [ ] Update namespaces
- [ ] Update file doc's

**Needs to pass on travis**:
- [ ]  - (PR coming)