Add aposthrophes back to dates

Commit 7318658017 removed apostrophes from dates and that caused
dibi to build different queries. Compare:
...WHERE `date_created` < '2014-02-21';
vs.
...WHERE `date_created` < 2014-02-21;
Merge pull request #129 from erikfercak/add_apostrophes_to_dates

Add aposthrophes back to dates