Add support for python 3 by requiring python3-openid in case of python3.
Also use 2to3 in case of python3.

Fixes #18.
Fixes #19.
diff --git a/setup.py b/setup.py
index 8074441..c0f7e89 100644
--- a/setup.py
+++ b/setup.py
@@ -12,6 +12,15 @@
   <http://github.com/mitsuhiko/flask-openid/zipball/master#egg=Flask-OpenID-dev>`_
 """
 from setuptools import setup
+import sys
+
+extra = {}
+install_requires = ['Flask>=0.10.1']
+if sys.version_info >= (3,):
+    install_requires.append('python3-openid>=2.0')
+    extra['use_2to3'] = True
+else:
+    install_requires.append('python-openid>=2.0')
 
 
 setup(
@@ -26,10 +35,7 @@
     py_modules=['flask_openid'],
     zip_safe=False,
     platforms='any',
-    install_requires=[
-        'Flask>=0.3',
-        'python-openid>=2.0'
-    ],
+    install_requires=install_requires,
     classifiers=[
         'Development Status :: 4 - Beta',
         'Environment :: Web Environment',
@@ -39,5 +45,6 @@
         'Programming Language :: Python',
         'Topic :: Internet :: WWW/HTTP :: Dynamic Content',
         'Topic :: Software Development :: Libraries :: Python Modules'
-    ]
+    ],
+    **extra
 )