Gilded comments

Two commits about comments. The first make `get_comments` part of `_methods` so it can be used by the Reddit object, similair to get_flair or get_banned. This change means helpers._get_section is now only used to generate non-subreddit listings as per it's docstring.

I also made `get_all_comments` simply give 'all' as the subreddit parameter in a call to `get_comments`. /comments and r/all/comments are identical and both pull from unsubscribed subreddits. But maybe we can't be sure this will always be this way and have to essentially duplicate `get_comments`?

The second adds `gilded_only` to both `get_all_comments` and `get_comments`.