Use runpy.run_path() or 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() doesn't set some globals like __file__ as import does,
which has to be done manually.
It also might be a bit slower for large body of code in settings file.

runpy.run_path() can only be used on python-2.7, but does not seem
to have any downsides.