setup: simplify the setup by removing the "tahoe dependencies" fake project

setup: simplify the setup by removing the "tahoe dependencies" fake project
Now we use "./setup.py develop" to ensure that changes to our source code are immediately used without requiring a "make" step.  This simplification will hopefully pave the way for easier py2exe and py2app, solving the "Unit tests test the installed version" bug (#145), and perhaps also #164 and #176.

This patch also conditionalizes the use of setuptools_darcs on the absence of a PKG-INFO file, which is part of fixing #263.