NetSparkleAppCast Parser Fix

* Added solution file for Visual Studio 2013.
* Fixed typo in filename and renamed NetSparkleAppCaseItem.cs to
NetSparkleAppCastItem.cs.
* Fixed bug in NetSparkleAppCast.Parse method which used an invalid
format string with DateTime.ParseExact when trying to parse the
publishing date in the AppCast.
DateTime Parser Fix

There was one more issue with the DateTime parsing, the pubDate field
uses an RFC 822 type date which has a four digit timezone without colon,
and only 'zzz' will parse that correctly, 'zz' will throw an error.
Merge pull request #4 from Sertelegger/master

NetSparkleAppCast Parser Fix