Add universal wheel support

As djangosaml2 is a pure Python library, it can be packaged as a
universal wheel.

Advantages of wheels

* Faster installation.
* Avoids arbitrary code execution for installation. (Avoids setup.py)
* Allows better caching for testing and continuous integration.
* Creates .pyc files as part of installation to ensure they match the python interpreter used.
* More consistent installs across platforms and machines.

When you'd normally run python setup.py sdist upload, run instead python
setup.py sdist bdist_wheel upload.

For more information on wheels, see:

http://pythonwheels.com/

Author: jdufresne

setup.cfg

@@ -0,0 +1,2 @@
+[bdist_wheel]
+universal = 1