BF: A mouse with many buttons could have no label for the logging message
BF: warning messages about dropped frames were crippling the MovieStim2
DOCS: info about P4N 2015
BF: fixes to gamma setting and also unsetting when win is deleted

gamma was not being set from the Window object because of the new
attributes system

Also, Window.__del__ was not being called because the openWindows
list kept a reference to it. That needs to be a weak reference.
FF: openWindwos list now uses weakrefs so be careful to use those during access
Merge pull request #727 from peircej/master

Fixes to Movie2 excessive warnings and also to gamma settings from monitorCenter