Fix installation (setup.py dependencies), add AppImage build script & update docs

[setaur]

Hi Scott! Thank you for this nice app, but it seems like it was not working from some time: #112 #111 #109

I've made a few updates to get it working again. Here is the summary of the changes:

• setup.py - Added missing dependencies so the app installs correctly.
• AppImage - Added a make-appimage.sh script used to generate AppImage file uploaded to the release section https://github.com/setaur/key-mon/releases/ of my fork page.
• Documentation - Updated README.rst with installation instructions (also added cool gif docs/keymon.gif). I changed only the beginning of the file.
• Updated RELEASE.rst

Feel free to merge all of these changes, or cherry-pick only the ones you find suitable.

Thanks again for the original work!