Fix #900: Print message that Python 3 is not supported and fail.

I've put this into PyInstaller/__init__.py, so *none* of the Pyinstaller-
tools will work with Python 3, not even archive-viewer nor makespec. If
this is not desired, we can move it into PyInstaller/build.by
Fix #910: Remove -o option from manual.
Update usage-options in man-page for build, make_comserver, makespec.