Added missing field from 1.7 migration
Fixed FK fields to work with Django 1.7 migrations

FilerFileField and FilerFolderField hard-coded the positional 'to' argument but Django migrations deconstruct for ForeignKey will pass this in via kwargs. Updated fields to use kwargs when hard-coding 'to' so migrations work.
Removed invalid kwarg before calling __init__

This was breaking since Field does not support the limit_choices_to argument
The line noise in file.py is just removing an unused variable 'other_widget'
Merge pull request #1 from jsma/feature/django-1.7

Added missing field from 1.7 migration