Update AuthModule for AG-Security

With the release of AeroGear Security 1.0.1 we got rid of the AeroGearUser type.

This has an impact on code that is login in against it:

The AeroGEarUser has a "username"
https://github.com/aerogear/aerogear-security/blob/1.0.0/src/main/java/org/jboss/aerogear/security/model/AeroGearUser.java#L26

This is gone, and the "replacement", the User/SimpleUser from AeroGEar (extends Agent) is using "loginName":

http://docs.jboss.org/picketlink/2/2.5.0.Beta3/api/org/picketlink/idm/model/Agent.html#getLoginName()

The iOS library needs to reflect this change in here:
https://github.com/aerogear/aerogear-ios/blob/matzew_afn121/AeroGear-iOS/AeroGear-iOS/security/AGRestAuthentication.m#L122