Add option and keybinding to keep zoom and viewport settings.

Patch by sdaau on IRC. Thanks!
New sort option: mtime

Sort filelist by modification time, newer files first. Useful for
perusing a directory of images by most recently added or changed.
Merge pull request #119 from guns/sort-mtime

New sort option: mtime
add a desktop file (closes #98)
applications/Makefile: fix clean target
support "feh -" to read from stdin. closes #118
Merge branch 'master' of git://github.com/derf/feh