Use execfile() instead of import machinery for --settings-file

Problem with import is that it:

 - Reuses already-imported module.

  For example, --settings-file=/etc/zerobin.py won't be imported,
  because sys.modules will already contain 'zerobin' entry.

 - Alters sys.path.

  Importing module from a heavily-populated directory may potentially
  cause next imports to run code from totally unexpected places.

Using execfile() does not seem to have any of these downsides.