Added asynchronous saving

Concerning #300 and #301 

Jenkins Testrun:
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/941/

MyProjectsActivityTest run (testImageCache is already failing on the master branch):
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-single-UI-test/634/

ProjectUpAndDownloadTest run:
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-single-UI-test/635/