BDD: Fix behat contexts inheritance

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:
- [x] Make Mink only inherited by Browser
- [x] Make an API context to be the main context for API's
- [x] Make an shared context to be used for Browser and API's
- [x] Update structure
- [x] Update doc with sentences in BrowserContext 
- [x] Move sentences interfaces to the right place
- [x] Update namespaces
- [x] Update file doc's

**Needs to pass on travis**:
- [x]  ezsystems/ezpublish-community#155 -- [Travis](https://travis-ci.org/ezsystems/ezpublish-community/builds/25686608)


**Summary:**
At the moment all the profiles (test suites) are extended by Mink, which is an extra layer for browser testing.
This means that the Rest API, the commands and Public API were inheriting that layer, and this shouldn't happen.

So for this isolation to happen, it's needed that:
- all browser contexts inherit mink
- all the others inherit behat
- common functionality to be added into both contexts

This was the main point here.

Also, since it was requested in  a previous PR, I've added the sentences to the methods in BrowserContext
Moved all sentence class definitions into a folder for easy access
And updated the namespaces + class names to reflect the structure