first stab at windows build details.

first stab at windows build details.

there are many and various fiddly details that were involved in this process
on mountain view.  This is a stripped down version of the build process used
there.  there's hence a good chance that one or two necessary details got
stripped down through the cracks.

this provides a py2exe setup.py to build a tahoe.exe and a tahoesvc.exe
the former is equivalent to bin/tahoe, but without the start/stop commands.
the latter is a windows service that instantiates a client whose basedir
is found in the registry.