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

[himdel]

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 :).

[dg]

Thank you!