Add SqlFunction support in addWhere

Add SqlFunction support in addWhere

This adds an optional $isExpression parameter in addWhere that
allows use of SqlFunctions in WHERE expressions:

Contact::get(FALSE)
  ->addWhere('last_name', '=', $last_name)
  ->addWhere('LOWER(first_name)', '=', "BINARY('ALICE')", TRUE)
  ->execute()->indexBy('id');

This also adds a few tests to the existing expression support for
ON and HAVING clauses as well as a new BINARY function.