GetStrings crashed if tags or categories were set, always returned empty.

Need to convert items one by one here, not just copy-paste GetString().
Make slugs have zero-padded dates. YYYY-MM-DD not Y-M-D.
Make default layout actually trigger.

The map value is interface{}, so comparing it to "" gives false for
unset values, and leads to pages not having any default style.
Merge pull request #13 from bazillion/master

Please pull bugfixes