DibiTranslator: respect %if blocks for %lmt and %ofs as well

Fixes issue #87.

In current dibi, something like 

    \dibi::query([
        'SELECT * FROM foo',
        '%if', false,
            '%lmt', 3,
            '%ofs', 5,
        '%end',
    ]);

yields `SELECT * FROM foo /* */ LIMIT 3 OFFSET 5`, which is probably not what you want.

This changes the translation of `%lmt` and `%ofs` to only set `->limit` and `->offset` when `->comment` is falsy.

The new behaviour is to generate `SELECT * FROM foo /* (limit 3) ... (offset 5) */`.
I would have preferred to generate proper SQL, but it would have been possible only with the MySQL*, Postgres and SQLite* drivers. The other drivers' `applyLimit` changes the whole SQL statement and thus would generate horrible stuff.

Thus, while I feel it's necessary to mention the limit/offset in the comment (because debugability), I think we should not pretend to generate valid SQL by making it `/* LIMIT 3 OFFSET 5 */` because since we can't make it valid always, we should be explicit about it not being valid.

But feel free to change it, obviously :).