port backend get_available() to use plain http instead of restfulclient and use API only authenticated

Binary package hint: software-center

The software-center-agent is changing:

> - We'll be using the view instead of the API call to access the public
> list of apps, as we can serve that over plain http. The url for the
> list of available apps is /apps/en/ubuntu/<series>/<architecture>/, for
> example http://sc.staging.ubuntu.com/apps/en/ubuntu/maverick/i386/.
> When you've switched the desktop app's code we'll be removing the
> application set from the API altogether.

This needs to be ported in the app.

Once that is done, the next step is to use the API only when authenticated via oauth and
remove using getForOAuthToken() but use getForMe() instead.