Implementing Wiki

[jamesrswift]

I've started implementing a controller for wiki objects. This PR is still very much a draft and work in progress, having still several things that will need doing before it is ready (some of which I will admit not to having enough knowledge of the codebase to do myself). I'll list these below:

• wiki.getDiscussions(). This returns a list of discussions about a given wiki page, and takes subreddit and page as arguments. This is in the form of a listing, which is arguably the most obtuse part of the Reddit API, and one I won't be able to do myself.
• Similarly, wiki.getRevisions(). This returns a list of recently changed wiki pages in the given subreddit.
• Once more, wiki.getPageRevisions(). As above, returns a list of recent changes but for a given page on a subreddit.
• wiki.getPage(). This returns the contents of a wiki page, but can take revision IDs to yield a diff if so desired. I imagine that this function should be implemented as internal, and expose two separate functions for the two different uses.
• I haven't started yet on any of the return typings, as the API documentation helpfully doesn't provide this. This will come in a later commit.
• No work has been done on implementing a wiki object class. This will require some design decisions on how to handle revisions, and pages, either separately or together.
• wiki.getSettings and wiki.changeSettings do not have the settings interface defined yet, I want to experiment whether the endpoint for changing settings accepts partial settings or if it needs all of them.

All the best :)

[jamesrswift]

Everything is just about completed, except wiki.getDiscussions( subreddit, page ). I don't seem able to use the endpoint without it returning an empty listing, so I'm not sure what type of data it even responds with. I have a hunch it responds with items of type Submission, but no idea for sure.

[jamesrswift]

Merged in changes related to #88 (see #91). Only thing left to implement is wiki.getDiscussions( subreddit, page ). I'm happy for this to be reviewed now though, and hopefully pulled.