Adds a few of my startup files that may be useful in future

For future reference, these could be placed in $HOME/gaffer/startup/gui for gaffer to pick up on start

* displays.py
** just basic example of adding some default aovs to the Displays presets list
* exportRibs.py
** example of looping over frames and writing out ribs for selected RendeManRender node
* setupScreenGrabSession.py
** most handy - is used to export python cmdsfile for use with screengrab app. that file will help recreate the current layout,
selection, etc