Add age_rating, community_rating and episode_length to APIv1

Following up on #261. With this, the remaining differences between `present_anime` methods are as following:

- v1 provides `status`, where v2 expects the client to calculate it from airing dates. Not an issue.
- v1 provides `url`, where v2 expects the client to put `https://hummingbird.me/anime/` and `slug` together. Not an issue, obviously.
- v1 has `title` and `alternate_title`, where v2 has `canonical_title`, `english_title` and `romaji_title`. As far as I'm aware, they end up transferring the same amount of information.
- v2 has `screencaps` and `youtube_trailer_id`. Not an issue, personally.

In other words, this is about as good as it gets without breaking backwards compatibility.