Store authentication cookie to be able to view purchased videos.

Hello.

This patch allows the plugin to view purchased videos.
The problem with them is that you need to be logged on when requesting information about video. In other words you need to supply proper cookie to the server.

So I will store cookie named "vimeo" during the authetication phase. And then put this cookie to cookiejar at next plugin invocation (because authentication is done only once).

There are several things I'm not sure about:
- proper place for the cookie - I'm storing it into settings (oauth tokens are stored there too)
- whether to put the cookie to plugin's cookie jar or just use it in particular fetchPage
- how to do cookie renewal - it seems the cookie is valid for about month - so how to request new cookie after that time ...

Anyways with this patch (and other two I've sent) I'm able to watch purchased videos.