Generate AppImage

[probonopd]

This PR, when merged, will compile this application on Travis CI upon each git push, and upload an AppImage to a temporary download URL on transfer.sh (available for 14 days). The download URL is toward the end of each Travis CI build log of each build (see below for how to set up automatic uploading to your GitHub Releases page).

For this to work, you need to enable Travis CI for your repository as described here prior to merging this, if you haven't already done so.

Providing an AppImage would have, among others, these advantages:

• Works for most Linux distributions (including Ubuntu, Fedora, openSUSE, CentOS, elementaryOS, Linux Mint, and others)
• One app = one file = super simple for users: just download one AppImage file, make it executable, and run
• No unpacking or installation necessary
• No root needed
• No system libraries changed
• Works out of the box, no installation of runtimes needed
• Optional desktop integration with appimaged
• Optional binary delta updates, e.g., for continuous builds (only download the binary diff) using AppImageUpdate
• Can optionally GPG2-sign your AppImages (inside the file)
• Works on Live ISOs
• Can use the same AppImages when dual-booting multiple distributions

Here is an overview of projects that are already distributing upstream-provided, official AppImages.

Please note: Instead of storing AppImage builds temporarily for 14 days each on transfer.sh, you could use GitHub Releases to store the binaries permanently. This way, they would be visible on the Releases page of your project. This is what I recommend. See https://docs.travis-ci.com/user/deployment/releases/. If you want to do this for continuous builds, also see https://github.com/probonopd/uploadtool.

If you have questions, AppImage developers are on #AppImage on irc.freenode.net.