Resolving parts of #688907:

Resolving parts of #688907:
- Replaced bootstrap shell script with Python script. This means
  standalone apps built with bundlebuilder will not work on MacOS < 10.1,
  since we depend (again) on an installed Python.
- Add a hack to set sys.executable; the bootstrap script does os.execve()
  with an argv[0] that's different from the actual Python executable
  (it has to match the CFBundleExecutable entry in the Info.plist to make
  the app work both from the Finder and the command line, and it has to be
  the bootstrap script), yet a proper sys.executable is needed to spawn
  auxiliary processes.