Added tests to verify static_proxy.

I added some tests to check different cases of the ```static_proxy``` view. This is related to: #924

Note that the ```static``` tag in ```django.contrib.staticfiles``` uses  ```STATICFILES_STORAGE``` to build the url [0], so I changed it to ```django.templatetags.static.static``` which concatenates the setting variable and the argument. 

All test pass with the latest commit https://github.com/stephenmcd/mezzanine/commit/68a6813edc714ea751d7a866ad57aeac223801e8

[0] https://docs.djangoproject.com/en/1.6/ref/templates/builtins/#std:templatetag-static (see the note)