title: Adding foreignKey widget to django-import-export
body: <p>I'm trying to import data to one of my models, but it's failing because I'm trying to upload the foreignKey Id, not the iterated number that import-export creates.</p>
  
  <p>models.py</p>
  
  <pre><code>from django.db import models
  from import_export import resources
  
  
  class School(models.Model):
      name = models.CharField(max_length=100)
      slug = models.CharField(max_length=100)
      school_id = models.IntegerField(unique=True)
  
  class Sol(models.Model):
      school_id = models.ForeignKey(School, to_field='school_id')
      name = models.CharField(max_length = 100)
      Al1EOC = models.DecimalField(max_digits=5, decimal_places=2)
      AL2EOC = models.DecimalField(max_digits=5, decimal_places=2)
  
  #Class for django-import-export
  class SolResource(resources.ModelResource):
      class Meta:
          model = Sol
  </code></pre>
  
  <p>My admin.py</p>
  
  <pre><code>from import_export.admin import ImportExportModelAdmin
  
  
  class SolAdmin(ImportExportModelAdmin):
      list_display = ('name', 'school_id')
      resources_class = SolResource
      pass
  
  admin.site.register(Sol, SolAdmin)
  </code></pre>
  
  <p>My data.csv</p>
  
  <pre><code>id, name, school_id, Al1EOC, AL2EOC
  ,'Main st school', 1238008, 12.9, 14.9
  </code></pre>
  
  <p>When I export the data from the SOL model, I get an iterated number for the school ID. I want the actual School ID - the one that holds the foreignKey relationship. And, subsequently, I need to upload data with that foreignKey number. I know the ForeignKey widget is the way to do it, but I don;t understand how it is implemented.</p>