Allow delegation of preferences so that external apps can consume tangerine

Tangerine should be controllable from external applications. Since tangerine is a non-interactive (daemon) application this specification mostly refers to delegating the control of tangerine preferences to an external app.

The settings that will be exposed will be:
    * what plugin to use (this will usually be implicit based on who is setting the prefs. For instance, if banshee is using the prefs interface then Banshee will be set as what to share).
    * share name
    * connected user limit
    * share password

I'm thinking that a dbus interface may be the best way to do this. This would allow any app which can use dbus can use tangerine.