Fix reader -> member issue in secureDB.

I believe the name of the parameters for the _security database is incorrect.

According to the documentation at [The Apache Site](http://wiki.apache.org/couchdb/Security_Features_Overview#Authorization], the names of the top-level entries should be 'admins' and 'members', not 'admins' and 'readers'.

I hope to try and actually create a test for this in the next couple of days, but please don't hold me to that.