Fix presentation-plugin

[sokra]

see http://stackoverflow.com/questions/11891949/how-to-find-cause-for-syntaxerror-json-parse-unexpected-character

• prefix jmpress to iframe messages.
• fix the security issues

One should only include the presentation-plugin for a presentation NOT in normal websites! (That's why it is named presentation-plugin :smile: )

[shama]

Thanks! :D