[MERGE] Api versioning infrastructure

[Robert Collins]

bzr-svn and bzr-gtk have been annoying me enough that I've scratch that
itch. Herewith a patch to export explicit API information from bzrlib,
with an initial exported api of just bzrlib. This is a starting position
only - we can go from here and add:
 - convenience functions that issue deprecations and hide the exceptions
 - a function that takes many versions and modules and checks them all
 - more precise API's centred around Tree, Branch, Repository, Transport
etc if we think its worth while. (I think it is; but can wait for
that :)).

-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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