Change bundling instructions to work with py2exe.

py2exe requires extra work to help it find modules, like unzipping
eggs and creating dummy __init__.py files.
Use py2exe in dist-exe target.

Make the dist-exe make target use p2exe rather than pyinstaller.
This removes some error handling and cleanup.
Use sys.executable to get script directory when frozen.

py2exe overwrites sys.path[0], and the paths to the registration
helpers end up wrong. If we are frozen, use dirname(sys.executable)
instead of sys.path[0].