setup: a new improved way to create tahoe executables

setup: a new improved way to create tahoe executables
Create the 'tahoe-script.py' file under the 'bin' directory. The 'tahoe-script.py' file is exactly the same as the 'tahoe-script.template' script except that the shebang line is rewritten to use our sys.executable for the interpreter. On Windows, create a tahoe.exe will execute it.  On non-Windows, make a symlink to it from 'tahoe'.  The tahoe.exe will be copied from the setuptools egg's cli.exe and this will work from a zip-safe and non-zip-safe setuptools egg.