Added introspection rule for Django South

I wanted to add my new CloudinaryField model field to the database using South, but South throws the error:

    ! Cannot freeze field 'myapp.mymodel.myimage'
    ! (this field has class cloudinary.models.CloudinaryField)
     
    ! South cannot introspect some fields; this is probably because they are custom
    ! fields. If they worked in 0.6 or below, this is because we have removed the
    ! models parser (it often broke things).
    ! To fix this, read http://south.aeracode.org/wiki/MyFieldsDontWork

I did this with the guidance of the South documentation: http://south.readthedocs.org/en/latest/customfields.html#extending-introspection

This will make the pycloudinary module "south aware"