New implementation of the automatic updater

This new updater makes requests directly into the GitHub API to identify the latest version, instead of requiring us to manually update the version files and check them it. It's also implemented as a separate module, allowing it to be run while the HearthStats Uploader is closed and thus avoid problems where the uploader isn't updated properly because it's in use.

Note that I haven't yet implemented most of the necessary error handling, so it will currently throw exceptions if there are no new releases published (which is the case right now...)
Prettier UI for the updater, more robust error handling
New logo, new name!
Added Windows support to the new updater.

Also cleaned up a comments and loig statements.
Made background image on updater larger to suit Windows UI
Added version number to the 'new version is available' popup
Updater can now handle releases for one platform only — ie we can release a Windows-only update without the OS X version downloading it.

The download of draft releases is now configured in the updater.properties file.
Update files downloaded by the updater are now deleted on startup.
Merge pull request #460 from gtch/updater

New implementation of the automatic updater

This is now working on both Windows and OS X, so it's ready for use.