Use the proxy model for Pin.submitter so Tastypie uses a correct model for its FKs

Tastypie's ModelResource makes use of the underlying Model class to figure out its fields. It obviously
breaks when there is inconsistency with models, which is rather rare, but possible to accomplish (which I've
proven with the previous commit).

pinry/pins/models.py

@@ -1,13 +1,14 @@
 from django.db import models
 from django.core.files import File
 from django.core.files.temp import NamedTemporaryFile
-from django.contrib.auth.models import User
 
 from taggit.managers import TaggableManager
 import urllib2
 import os
 from PIL import Image
 
+from pinry.core.models import User
+
 
 class Pin(models.Model):
     submitter = models.ForeignKey(User)