Add a QuerySafeChoiceField

[yeago]

I run into situations often where users see a multi-select box as a chance to select all options within that box. This wouldn't be a problem, except SQL IN(...) can be rather heinous in some database (slow query log one time reported 77k+ rows were affected by a queryset that only got about 2k queries)

http://dpaste.com/hold/120201/

Obviously, some other result might be preferable than the above paste.. Perhaps replacing queries executed that way with the less abbraisive is_null=False?

[alex]

Fixed in 83c683b