Preliminary wiki support

Everything is here, except wiki page revision diffs which i don't think have support in the public api. Everything is in a new Wiki object in the Subreddit class.

The only modification to existing code is in the Thing class.  I had to add parsing by generic type (Parse<T>) so I could get Listing support for Things that aren't "Things" like page revisions.  Slight change to Listing to call the generic Parse function. There may have been a easier/better way to do this.