Parallelize fetch and parse of data in RssService

Parallelization of fetching and parsing of data now working on Windows 8
and Windows Phone.  In a previous changeset
(https://github.com/roschler/XPlatformCloudKit/commit/58f5432073f779d8ff68f8074f0ca5f9e0073d58),
the parallel portion would hang on Windows Phone.