Appcast item - parse additional items from the update feed

[jozefizso]

The update feed may have additional elements which are not yet parsed by the Appcast item SUAppcastItem class.

Additional field to parse:

• displayVersionString - value of the <sparkle:shortVersionString> element, or the sparkle:shortVersionString attribute from the <enclosure> element.
• date - value of the <pubDate> element.
• installationType - value of the sparkle:installationType attribute in the <enclosure> element.

Sample update item

<item>
    <title>Version 1.4 (241)</title>
    <description>Latest app release.</description>
    <pubDate>Sat, 26 Jul 2014 15:20:11 +0000</pubDate>
    <sparkle:version>241</sparkle:version>
    <sparkle:shortVersionString>1.4</sparkle:shortVersionString>
    <enclosure url="http://localhost:1337/Sparkle_Test_App.zip" sparkle:installationType="package" />
</item>