Move create_playlist from Channel to Account

According to YouTube API documentation, an account can create a
playlist without passing any information about the channel that
the playlist will belong to.

The playlist will always belong to the account’s channel, so
it is correct to have `account.create_playlist`, rather than
`channel.create_playlist`.
Merge pull request #87 from Fullscreen/move-create-playlists

Move create_playlist from Channel to Account