Make 'powerline' a namespace package

This will allow third-party plugins to use the 'powerline' namespace for
contributed segments.

Refs #3.

powerline/__init__.py

@@ -0,0 +1,5 @@
+try:
+	__import__('pkg_resources').declare_namespace(__name__)
+except ImportError:
+	from pkgutil import extend_path
+	__path__ = extend_path(__path__, __name__)

setup.py

@@ -33,4 +33,5 @@ setup(name='Powerline',
 			'Sphinx',
 			],
 		},
+	namespace_packages=['powerline'],
 	)