Cleanups concerning api level movement

Due to the switch to the lower api level 10 some changes were necessary.
These changes are now fixed - in the code base. We now need to test the
ui and its look and feel on newer devices, to make sure we don't miss
relevant features.
Add remove listener to the context, fix wrong usage of tree set
Various work in progress

* Use resources for message strings
* Add configuration within shared preferences
* Make the configuration activity load and save
* Fix missing onDestroy on parent within main activity
* Use the contact backend for circulating over contacts and don't issue
  a array out of bounds exception
* Put some initial example code into view strategies