ZarafaBridge: add method to get Webaccess settings string

This will be useful later to get the user's shared ("other") stores.
ZarafaBridge: add support for shared addressbooks

Based on code by @erwint:
  https://github.com/erwint/sabre-zarafa/
Split off the Webaccess settings into separate class

Separation of concerns. The ZarafaBridge class is getting a bit bloated.