Support formulas inside select()

• external.prepare for Model and Property became Expr instead of dict.

• Expr now has it's own unparse and preserves exact expression representation.

• Sql backend now supports formulas in select().

• count() now must be inside select(), but only for Sql backend.

• Property.external no longer can be a list, if you need more than one value, use prepare.

Fixes #39 (closed)