Add documentation for application plugins.
Add documentation for application plugins.

Add section about Django by jeweber.
Add section about ownCloud.

Closes #558

jh23453/privacyidea,wheldom01/privacyidea,wheldom01/privacyidea,wheldom01/privacyidea,jh23453/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,wheldom01/privacyidea,wheldom01/privacyidea,privacyidea/privacyidea,jh23453/privacyidea,wheldom01/privacyidea,privacyidea/privacyidea,jh23453/privacyidea,jh23453/privacyidea,jh23453/privacyidea