Put the headlines of the commits since last upload into the description

[probonopd]

Instead of seeing:

Travis CI build log: https://travis-ci.org/AppImage/AppImageUpdate/builds/302102315/

it would be cool to see the headlines of the commits since the last upload as part of the description (kind of a mini changelog):

Travis CI build log: https://travis-ci.org/AppImage/AppImageUpdate/builds/302102315/

• Show progress in MiB as feature demo for #40
• Implement #40
• Describe variable

[TheAssassin]

When I last tried to add a similar feature, I broke the script totally. Bash is just not the language for this job. I think you should add it to the "rewrite in C++" milestone.