Us/1380/sentry

Raven support for sentry error reporting
Independent of my previous comment, this makes a default config with raven activated with dummy config... I think is not a good approach.
Raven is mainly used by our production enviroment, but not should be activated on each standalone installation. (Like analytics) Because of it, I think that should be out of the example settings.

Extra settings will be documented in future on the main documentation, but now is not the main priority.