Add RSS server functionality

Add an RSS server as suggested by Nick Sharples, loosely based on his
original POC code, with changes including the following:

* Produce a single combined feed of all downloaded episodes instead of a
  separate feed for each programme.
* Fix FxCop and StyleCop warnings.
* Add itunes:duration and itunes:image tags for each episode.
* General tidying up, streamlining and more robust error handling.
* Ability to handle more than one connection at a time.
* Handle line-breaks in episode descriptions correctly.
* Expose settings in the options dialog.

This resolves #197.