Revive the -r option.

The -r (read list of image files/folders from stdin) went missing somewhere between the 1.0 and 2.0-pre release. This re-adds it (mostly with code taken from the 1.0 source). See issue numbers 21 and 22 for discussion.
Updated Readme with new -r option, added preed to contributors list
Do not exit if the first image given to -r fails to load

The image loader initialization fails if none of the images successfully
loads. Until now, it then printed an error message & quit. With this
change, it just silently fails. Any error messages are outputted only
when it is clear that all images from the command line and -r have been
processed.
Typo in the README
Use a weak reference in the image removal callback

This is required for "-rl" invokations of pqiv
Use a mutex for bostree access

The image loader thread requires this. Maybe I'll move the thread safety
into bostree one day.
Merge branch 'master' into redraw