Replace `reddits` with `subreddits` where appropriate.

Corresponds to this change: http://www.reddit.com/r/changelog/comments/1a8i4n/reddit_change_everywhere_using_reddit_to_refer_to/

Deprecations:

* `get_my_subreddits` replaces the now deprecated `get_my_reddits`.
* `get_popular_subreddits` replaces the now deprecated `get_popular_reddits`.