Update notifications about buttercup-desktop

[perry-mitchell]

The auto updater looks shakey, so I suggest the following:

Store a JSON file on the server with the current version and the URL to the installer. The file could look like this:

{
    "version0": {
        "version": "0.2.3",
        "title": "Buttercup alpha 0.2.3",
        "importance": "normal",
        "readmore": "http://read.more/index.html",
        "installers": {
            "mac": "http://download.com/buttercup-0.2.3.dmg",
            "win32": "http://download.com/buttercup-0.2.3-x86.exe",
            "win64": "http://download.com/buttercup-0.2.3-x64.exe",
            "linux32-deb": "http://download.com/buttercup-0.2.3-x86.deb",
            "linux64-deb": "http://download.com/buttercup-0.2.3-x64.deb"
        }
    }
}

[perry-mitchell]

URL: http://buttercup.pw/update.json

Importances: "normal", "important", "critical"

readmore: An optional property (may not be present) to provide a link to a notice (URL).