Last-ditch fix for missing presentation images

We can only get here if all of the files for the presentation are
*completely* missing. One thing that can cause this is if the
presentation filename starts with a '.' character - the presentation
files aren't correctly archived then.
Merge pull request #290 from kepstin/presentation-missing-slide

Last-ditch fix for missing presentation images
created a new locale string for the chat module
Merge pull request #291 from SenecaCDOT-BigBlueButton/add-another-string

created a new locale string for the chat module
minor bug fix in old stress testing script
 - fix https://code.google.com/p/bigbluebutton/issues/detail?id=1680
Changing lables for 0.81
Merge branch 'master' of github.com:bigbluebutton/bigbluebutton
Minor update to bbb-conf
Merge branch 'master' into webrtc_support