get rid of tahoe.exe launcher

[zooko]

There is a binary (cli.exe) in the Tahoe-LAFS source tree under revision control. It is built from launcher.c in zetuptoolz:

​http://tahoe-lafs.org/trac/zetuptoolz/browser/launcher.c

source:setup.py​ copies it to bin\tahoe.exe when building.

There are several reasons to get rid of this launcher:

• it mangles any non-ASCII arguments, preventing us from fixing #565 on Windows;
• it adds a small amount of overhead to running CLI commands;
• it's unnecessary complexity;
• it isn't in the spirit of open source to have a binary that is not compiled from source as part of our build process -- getting rid of it fixes this without complicating the build or requiring a C compiler;
• a Python script called tahoe.py can be run just fine on Windows as tahoe, provided that the PATHEXT environment variable includes ".py".

[davidsarah]

Attaching my work-in-progress for Unicode argument support on Windows and removal of tahoe.exe, so that it doesn't get lost.