Create GitHub releases and adjust the auto-updater

[xeruf]

GitHub releases are probably the way to go for the future, but they should still work properly with the auto-updater.
Furthermore, they should be fit into the gradle "release" task to automatically publish a release using the GitHub Releases API.

[fwcd]

@Xerus2000 This would integrate nicely with #23 (which could automatically deploy to the releases every time you push)

[xeruf]

But I don't really want to deploy whenever I push. Every Release will be explicitly created by calling the release task.

[xeruf]

I can already publish releases on the github-releases branch using gradle githubRelease. Now the application needs to be patched so it fetches updates from the GitHub Releases.

[xeruf]

Tasks:

• Do Github releases via Travis CI
• Patch the Auto-updater to download new releases from Github

This GitHub API route can then be used for the updating part: https://api.github.com/repos/Xerus2000/monsterutilities/releases
It returns the releases in JSON format (there is already a JSON parser in the application anyways) in reverse chronological order, just as we need it.

[xeruf]

Continued in #53