Setup Travis CI for build and deploy AppImage binary for Linux

Compile this application on [Travis CI](https://travis-ci.org/) upon each `git push`, and upload an [AppImage](http://appimage.org/) to your GitHub Releases page.

Providing an [AppImage](http://appimage.org/) would have, among others, these advantages:

- Applications packaged as an AppImage can run on many 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](http://discourse.appimage.org/t/how-to-make-an-appimage-executable/80), 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
- Can be listed in the [AppImageHub](https://appimage.github.io/apps) central directory of available AppImages
- Can double as a self-extracting compressed archive with the `--appimage-extract` parameter

[Here is an overview](https://appimage.github.io/apps) of projects that are already distributing upstream-provided, official AppImages.

### TODO

1. https://github.com/appimage/appimagekit
1.1 https://github.com/probonopd/linuxdeployqt
1.2 https://github.com/probonopd/uploadtool

**PLEASE NOTE:** For this to work, you need to enable Travis CI for your repository as [described here](https://travis-ci.org/getting_started) **prior to merging this**, if you haven't already done so. Also, You need to set up `GITHUB_TOKEN` in Travis CI for this to work; please see https://github.com/probonopd/uploadtool.

If you have questions, AppImage developers (i.e. @probonopd and other) are on #AppImage on irc.freenode.net.