Add Adobe Photoshop CS6

I've added an cask for Photoshop CS, in spite of the the fact that I could not determine how to install the app automatically. Also, Adobe provides an uninstaller application (/Applications/Adobe Photoshop CS6/Uninstall Adobe Photoshop CS6), but I didn't create an uninstall stanza for it because it's a GUI uninstaller that can't be run from the command line.