This commit allows to edit an event via the API

When an event is submitted with the same urlfriendly name the event is
updated instead of newly created.
Adds PUT-Method to EventsController

That way events can also be edited via the API.
Fixes minor whitespace issue
Adds Event-Editing plus frisby-tests

This commit adds frisby-tests to check whether adding or editing events
does work as one would expect. Events can of course only be edited, when
the user in question is registered as host of the event.
removing some functions that are no longer used by this PR
use existing methods to check for user permissions
only use incoming fields if they are supplied, and don't initalise with existing data - only changed, supplied fields are pushed into the update statement, which uses the verboseFields list to work out which fields there are and what their db column names should be
do not update fields that are not supplied, but do accept empty values in fields and store those.  Also fix Location Url
remove the frisby tests from this branch, we don't have a way of running write-safe tests yet - this needs to be a separate PR
Merge branch 'heiglandreas-feature/addEditPossibility'