Delta updates for AppImages

[probono]

Working on delta updates for AppImages which means that updating the
Subsurface AppImage, e.g., from 4.5.1.449 to 4.5.1.464, requires
downloading just 0.7% of the AppImage, as the other 99.3% have not
changed and can be reused.

Testers and feedback welcome.

# Make sure curl is installed on your local machine
# (this dependency will go away)
which curl

# Get Subsurface 449 AppImage
wget https://bintray.com/artifact/download/probono/AppImages/Subsurface-4.5.1.449-x86_64.AppImage
chmod a+x Subsurface-4.5.1.449-x86_64.AppImage

# Run it
./Subsurface-4.5.1.449-x86_64.AppImage

# Get the updater
wget https://bintray.com/artifact/download/probono/AppImages/AppImageUpdate-20151218-x86_64.AppImage
chmod a+x AppImageUpdate-20151218-x86_64.AppImage

# Update Subsurface AppImage to the latest version
./AppImageUpdate-20151218-x86_64.AppImage ./Subsurface-4.5.1.449-x86_64.AppImage

# Look there is a newer version! Run it
NEW=$(ls Subsurface-* | sort -V | tail -n 1)
./$NEW

More information and bugtracker here:
https://github.com/probonopd/AppImageKit/tree/master/AppImageUpdate.AppDir