Fix use of historical ForeignKey name to use 'name' instead of 'name_id'

- When Historical* instance fields are accessed, use Field.name instead
  of Field.attname. For ForeignKeys this is sort of a monkey patch
  since the db name does not match the db name of the original model
  and for consistency it probably should be the same - 'fieldname_id',
  instead of just 'fieldname'.

  Only tested to work under django==1.4.5. Does not work under with
  django==1.3.1.