Allow to pass DSQL expression + condition + value

Very useful in situations like this:

```
// TASK: select distinct clients which have at least one unpaid invoice

$m = $this->add('Model_Client');
$m->_dsql()->option('DISTINCT');

// join invoice table (1:n relation)
$j = $m->leftJoin('invoice.client_id', 'id');

// this way it worked already, but only for DSQL->where()
$m->_dsql()->where($j->fieldExpr('sum_unpaid'), '>', 0);

// but model addCondition passed to its _dsql->where() only first parameter if
// its DSQL expression itself and not condition and value parameters
$m->addCondition($j->fieldExpr('sum_unpaid'), '>', 0);
```
Merge pull request #526 from DarkSide666/patch-12

Allow to pass DSQL expression + condition + value