Switch to setuptools and use install_requires

This fixes joh#17 and allows dependencies like pyinotify to be installed
automatically.

setup.py

@@ -1,15 +1,13 @@
-#!/usr/bin/env python
-
-from distutils.core import setup
+from setuptools import setup
 
 setup(name='when-changed',
-      version='0.2.0',
+      version='0.2.1',
       description='Run a command when a file is changed',
       author='Johannes H. Jensen',
       author_email='[email protected]',
       url='https://github.com/joh/when-changed',
       packages=['whenchanged'],
       scripts=['when-changed'],
-      requires=['pyinotify'],
+      install_requires=['pyinotify'],
       license='BSD'
       )