fixed challenge image error
Merge branch 'master' of https://github.com/Maddoc42/Ferienakademie2014Client

Conflicts:
	nEVERrEST/app/src/main/java/de/ferienakademie/neverrest/view/ChallengeActivity.java
	nEVERrEST/app/src/main/java/de/ferienakademie/neverrest/view/FindChallengesActivity.java