sqldiff | Show differences for not managed tables

When meta.managed == False was set in one model, sqldiff was considering
the model but not its fields. The output of sqldiff showed that all columns of
the table should be dropped in the database.
Now we show the differences from db to its associated django models.
sqldiff | Fixed bug with --output_text option and notnull-differ text

Using the --output_text option a bug was raised when a notnull-differ difference
was detected.
The previous output text message for a notnull-differ difference was
"field '%(1)s' null differ: db='%(3)s', model='%(2)s'".
However only 2 parameters were available in diff_args (l.488) ; the name of the model
and whether you should 'SET' or 'DROP' the NULL contraint in the database.
Merge pull request #535 from bsvetchine/master

sqldiff | Show differences for not managed tables