Refactoring of all BoolMonads: now BoolMonad.getsql() always returns a list of expressions just like other monads do
bool as a separate type
Add tests for bool type
least(a, b, c) and greatest(a, b, c) are used instead of min(a, b, c) and max(a, b, c) in all dbproviders except sqlite
bool to int conversion in PostgreSQL inside greatest/least
Translation of 'if expression'
Aggregation bug fixed (unnecessary joins were removed)
Use 'power' function for all db providers instead of '**'
test of 'min' with several arguments was fixed
Explicit casting of bool to int when necessary in PostgreSQL
Merge master to orm: bug fixes