Added 'embed finders'. Cleaned up wagtail embeds

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