fix possible NPE in ChallengeInfo
Merge branch 'unstable' of github.com:stestaub/drallo-app into unstable

# By Sergey
# Via Sergey
* 'unstable' of github.com:stestaub/drallo-app:
  merge
  282 done
  253 - add default image for drallo
  282 add new buttons
changed TargetFrameworkProfile in DAL-project to Profile136. This is necessary due to Xamarin's newest updates (stable-channel)
288 fixed
Merge branch 'unstable' of github.com:stestaub/drallo-app into unstable