Add a fieldlookup that accepts a callable to test the attribute.
Make QueryLists return QueryLists when combining with +

Previously, ql1 + ql2 would yield a list. This meant that there wasn't a reasonable way to combine two independent querylists and filter the data. This change makes it so that instead of a list, a new QueryList is returned.
Updated changelog.
Slight docstring clarification
Merge pull request #4 from thomasw/fix_magic_methods

Fix magic methods