BDD: Update structure for the inheritance fix

This belongs to a inheritance fix that involve:
- [x] ezsystems/BehatBundle#1
- [x] ezsystems/ezpublish-community#154
- [x] ezsystems/ezpublish-kernel#872 (EzPublishLegacyBundle and EzPublishRestBundle)
- [x] ezsystems/DemoBundle#90

Todo:
Both:
- [x] ~~move everything into an BddTest folder (so that the Feature and Context folder be on the same level)~~
- [x] Update main class name to reflect bundle
- [x] Update namespaces

Only for LegacyBundle:
- [x] Move SetupWizard sub context, into a main context

Only for RestBundle:
- [x] Change Error sub context class name to Exception
- [x] Add doc into RestContext methods (for the sentences implementations)

**Needs to pass on travis**:
- [x]  ezsystems/ezpublish-community#163