Integrate presence system-wide

A presence system, such as Telepathy, can be used to give information to the system, such as current location, what the user is currently doing, etc. and these could be looked for and acted upon, such as toning down the intensity of notifications when the user is set to "Busy" and increasing power management when set to "Away" for instance. Also some presence information can be set automatically, for instance a "Location" attribute could be set based on known Wifi networks within range. This spec proposes a discussion of possible ways presence can be integrated and a review of the feasability of each idea.