Don't invoke get_user_model on init (closes #752)

Squashed commit of the following:

commit 20ddacc4dcc38f305aade8ea28028cd0882aead5
Author: Raymond Penners <raymond.penners@intenct.nl>
Date:   Sat Nov 8 13:34:38 2014 +0100

    Integrated get_user_model related changes (not on import)

commit ccece672af2eedf9d2ada92d410c92d352de409b
Merge: d3b29a9 becc704
Author: Raymond Penners <raymond.penners@intenct.nl>
Date:   Sat Nov 8 13:11:07 2014 +0100

    Merge remote-tracking branch 'entequak/feature/django1.7' into django1.7

commit becc704800b39fdc86d93f8dcde20abf0088877b
Author: Christopher Grebs <cg@webshox.org>
Date:   Sun Nov 2 14:19:10 2014 +0100

    Remove the requirement for SILENCED_SYSTEM_CHECKS, this is actually quite silly but... I don't know any better way to support all Django versions 1.4-1.7

commit 9f50d7b18202294375bc6a249240c6bbfbe7e859
Author: Christopher Grebs <cg@webshox.org>
Date:   Sun Nov 2 13:24:16 2014 +0100

    finalize comment

commit b92100510f4f43405a66b9ba08fabf32e9cdc65c
Author: Christopher Grebs <cg@webshox.org>
Date:   Sun Nov 2 13:21:47 2014 +0100

    Bonus, it works just fine on Python 3.4 :)

commit 8b8757ec0073c76450d75ff005f1ae28fb342f5f
Author: Christopher Grebs <cg@webshox.org>
Date:   Sun Nov 2 13:19:34 2014 +0100

    Avoid calling get_user_model on import time. Restores Django 1.7 compatibility.

    The thing is, on import-time django.setup() was not called which leads
    to errors if you're calling get_user_model on Django 1.7.