Fix #16 Now the multiselectfield could be a readonly.

This solution is a little dirty, please if this does not work (in some case) tell me something

CHANGES.rst

@@ -3,10 +3,12 @@
 
 * Support to Django 1.7 (I'm sorry to the delay)
 * Adding get_FIELD_list function
+* Fix an error when a MultiSelectField was reandonly at the admin site
 * Thanks to:
     * `Hernil <https://github.com/hernil>`_
     * `Vasyl Stanislavchuk <https://github.com/vasyabigi>`_
     * `Litchfield <https://github.com/litchfield/>`_
+    * `Chris-erickson <https://github.com/chris-erickson>`_
 
 0.1.2 (2014-04-04)
 ------------------

multiselectfield/db/fields.py

@@ -57,6 +57,10 @@ class MultiSelectField(models.CharField):
         if self.max_choices is not None:
             self.validators.append(MaxChoicesValidator(self.max_choices))
 
+    @property
+    def flatchoices(self):
+        return None
+
     def get_choices_default(self):
         return self.get_choices(include_blank=False)