title: django-import-export ForeignKeyWidget doesn't do lookup
body: <p>I'm trying to import an xls file with django-import-export. </p>
  
  <p>This is what I have:</p>
  
  <pre><code>class CourseResource(resources.ModelResource):
  
     course_number = fields.Field(column_name='Kursnummer', attribute='course_number', widget=CharWidget())
  
     school_year = fields.Field(column_name='Schuljahr', attribute='school_year',
                            widget=ForeignKeyWidget(model=SchoolYear, field='name'))
  
  
     #other fields....
  
     class Meta:
         model = Course
         exclude = ('students')
         export_order = ('id', 'course_number', 'school_year')
  </code></pre>
  
  <p>The SchoolYear model:</p>
  
  <pre><code>class SchoolYear(models.Model):
     name = models.CharField(max_length=25, unique=True)
     start_date = models.DateField(verbose_name="Beginn")
     end_date = models.DateField(verbose_name="Ende")
  
     def __str__(self):
         return self.name
  
     def __unicode__(self):
         return self.name
  </code></pre>
  
  <p>The Course model:</p>
  
  <pre><code>class Course(models.Model):
  
     course_number = models.CharField(max_length=25, verbose_name="Kursnummer")
     school_year = models.ForeignKey(SchoolYear, default=30, verbose_name="Schuljahr")
  
     #some other fields...
  
     students = models.ManyToManyField(Student, verbose_name="Schüler", null=True, blank=True, related_name="courses")
  
     class Meta:
         unique_together = ('course_number', 'school_year',)
         verbose_name = "Kurs"
         verbose_name_plural = "Kurse"
         managed = True
  
     def __str__(self):
         return "%s (%s)" % (self.course_number, self.school_year)
  </code></pre>
  
  <p>An example row in my excel file looks like this (second column is the course number, third column is the name of the school year, then some more field): </p>
  
  <pre><code>... 7aMa 2015/2016 ...
  </code></pre>
  
  <p>When I try to import the file in the django admin, I get this error message:</p>
  
  <pre><code>Row number: 1 - Column 'school_year': invalid literal for int() with base 10: '2015/2016' [...]
  Traceback (most recent call last):
  File "/Users/amishelli/Dropbox/IMS/ims2/import_export/resources.py", line 441, in import_row
  self.import_obj(instance, row, dry_run)
  File "/Users/amishelli/Dropbox/IMS/ims2/import_export/resources.py",    line 300, in import_obj
  self.import_field(field, obj, data)
  File "/Users/amishelli/Dropbox/IMS/ims2/import_export/resources.py", line 290, in import_field
  field.save(obj, data)
  File "/Users/amishelli/Dropbox/IMS/ims2/import_export/fields.py", line 112, in save
  setattr(obj, attrs[-1], self.clean(data))
  File "/Users/amishelli/Dropbox/IMS/ims2/import_export/fields.py", line 68, in clean
   raise ValueError("Column '%s': %s" % (self.column_name, e))
   ValueError: Column 'school_year': invalid literal for int() with base 10: '2015/2016'
  </code></pre>
  
  <p>I presume the lookup of the ForeignKey for SchoolYear doesn't work. What am I doing wrong? </p>