Users and administrators can set how long an URL can be before it's
shortened, and how long a notice can be before all its URLs are
shortened. They can also turn off shortening altogether.

Squashed commit of the following:

commit d136b390
Author: Evan Prodromou <[email protected]>
Date:   Mon Apr 26 02:39:00 2010 -0400

    use site and user settings to determine when to shorten URLs

commit 1e1c851f
Author: Evan Prodromou <[email protected]>
Date:   Mon Apr 26 02:38:40 2010 -0400

    add a method to force shortening URLs

commit 4d29ca0b
Author: Evan Prodromou <[email protected]>
Date:   Mon Apr 26 02:37:41 2010 -0400

    static method for getting best URL shortening service

commit a9c6a3ba
Author: Evan Prodromou <[email protected]>
Date:   Mon Apr 26 02:37:11 2010 -0400

    allow 0 in numeric entries in othersettings

commit 767ff2f7
Author: Evan Prodromou <[email protected]>
Date:   Mon Apr 26 02:36:46 2010 -0400

    allow 0 or blank string in inputs

commit 1e21af42
Author: Evan Prodromou <[email protected]>
Date:   Mon Apr 26 02:01:11 2010 -0400

    add more URL-shortening options to othersettings

commit 869a6be0
Author: Evan Prodromou <[email protected]>
Date:   Sat Apr 24 14:22:51 2010 -0400

    move url shortener superclass to lib from plugin

commit 9c0c9863
Author: Evan Prodromou <[email protected]>
Date:   Sat Apr 24 14:20:28 2010 -0400

    documentation and whitespace on UrlShortenerPlugin

commit 7a1dd579
Author: Evan Prodromou <[email protected]>
Date:   Sat Apr 24 14:05:46 2010 -0400

    add defaults for URL shortening

commit d259c37a
Author: Evan Prodromou <[email protected]>
Date:   Sat Apr 24 13:40:10 2010 -0400

    Add User_urlshortener_prefs

    Add a table for URL shortener prefs, a corresponding class, and the
    correct mumbo-jumbo in statusnet.ini to make everything work.