Fix bug when creating temporary URLs

Pythons HMAC does not accept unicode, therefore converting key and hmac body to
string (to be compatible with Python 2.x). Python 3 needs a byte conversion.

See also: http://bugs.python.org/issue5285

Author: cschwede

setup.py

@@ -8,7 +8,7 @@
 
 setup(
     name='django-swiftbrowser',
-    version='0.21',
+    version='0.22',
     packages=['swiftbrowser'],
     include_package_data=True,
     license='Apache License (2.0)',