Accept `part` in the `where` clause of Channels

This is especially important when retrieving a list of partnered
channels for a content owner, since it allows to fetch in one query
not just the snippet and the status, but also the statistics set.

For instance, the following code:

```ruby
content_owner = Yt::ContentOwner.new owner_name: 'CMSname', access_token: 'ya29.1.ABCDEFGHIJ'
content_owner.partnered_channels.where(part: 'statistics').map &:subscriber_count
```

generated **one query for each partnered channel** before this commit, but now
just generates **one query**.
Accept `where` in ViewerPercentages collection.

This prevents N+1 queries when retrieving viewer percentages for
multiple partnered channels at once.
Delegate `.each_cons` and `.each_slice` to `list`.

Required to write code like https://github.com/Fullscreen/yt/pull/97#issuecomment-58424501
Only set `mine: true` on `account.channels`

This small changes allow to use `Yt::Collections::Channels` in the same
way as `Yt::Collections::Videos`, by passing a list of IDs.
`partnered_channels.includes :viewer_percentages`

`content_owner.partnered_channels` now accepts a chainable `includes`
method that, when called with `:viewer_percentages`, eagerly loads all
the viewer percentages of the partnered channels, so they can be
accessed without N+1 queries.
Merge branch 'allow-statistics-part-on-channels'