add a formfactor session variable which is autodetected but can also manually overwritten via a get variable.
currently we have:
         * default -> the normal desktop browser interface
         * mobile -> interface for smartphones
         * tablet -> interface for tablets
         * standalone -> the default interface but without header, footer and sidebar. just the application. useful to use just a specific app on the desktop in a standalone window.
In the future we should adapt the userinterface to the specific formfactor.

remove error output if symlink already exists. this can happen if a user removed config.php to trigger the setup wizzard

fixes file upload

having a broken web/card/caldav is much better as having no ownCloud at all :)

OC_Image::__invoke didn't return anything thus causing contacts/thumbnail.php to spit out lots of error messages.

Added method 'valid()' to tell if image contains a valid resource.
Renamed imageResource() to resource().