Changes to be compatible with django 1.5. custom user model

Hi, I have made new branch with clean commit history. I think that this is really good idea and necessary in order to remain compatible with django 1.5. This pull request solves problems with registering custom user model, which is available since django 1.5.

There are two new settings variables.

`SOCIALREGISTRATION_USER_FUNCTION` which points to the new-type function. The function fills up necessary fields in the user model and returns tuple: `(user, profile)`

`SOCIALREGISTRATION_SEAMLESS_SETUP` is just rename for `SOCIALREGISTRATION_GENERATE_USERNAME` which does not make sense if/when the old function will be removed. Both variables are kept and used as dispatcher.

The change is backward compatible.