[MERGE] Extends the launchpad plugin's implementation of lp spec urls

[Ian Clatworthy]

Tim Penhey wrote:
> Extends the launchpad plugin to handle lp urls of the form 
> lp://staging/, lp://demo/, lp://dev/ to use the appropriate 
> launchpad instance to do the resolution of the branch identities.
> 
> This is primarily of use to Launchpad developers, but can also
> be used by other users who want to try out Launchpad as
> a branch location without messing up their public Launchpad
> account.  Branches that are pushed to the staging environment
> have an expected lifetime of one day.

bb:tweak

The information above needs to be added to NEWS.

If you need me to merge this (after tweaks), let me know.

Ian C.