Allow more options for authenticated users

[Michał Sawicz]

The authentication API is now available, the plugin should incorporate that and allow more interaction, like:

- list user's videos
- list user's contacts
- list contact's videos
- list contact's liked videos
- list contact's contacts
- etc.
- list user's subscribed channels instead of those defined in the config file
- list groups?
- etc.

I'd like someone from the outside to decide what might, and what might not, be useful. Just browse through the Vimeo site and see what's nice there and is missing in the plugin.