Accept upper or lower case auth arguments in OPTIONS

[mhluongo]

See the issues that came up http://stackoverflow.com/questions/21812824/using-neo4django-with-graphenedb/21815988?noredirect=1#comment33016586_21815988 . Django custom is typically to use all-caps for settings like this, so accept both.

Refs #224.

[stephenmuss]

I came across this myself when trying to get going with graphendb. However, digging into the source was enough to work out what I needed to do.

To be honest, simply documenting OPTIONS in the docs would be enough. If it were clear to people what was happening here the wouldn't really be a problem.