Added 'embed finders'. Cleaned up wagtail embeds

- Added WAGTAILEMBEDS_EMBED_FINDER setting
- If this setting is not set, it will fall back to embedly. If EMBEDLY_KEY is not set, it will fall back again to oembed.
- Restructured and cleaned up wagtailembeds a bit
- Added some unit tests

Serafeim, Please let me know if you're happy with my changes.