Make '0' possible as a field value. Validate for empty strings instead of empty()

This change makes it possible for options to have '0' as the field value. Validation now checks against empty strings instead of empty().

Please make the necessary changes if some things are conflicting.

Thanks!
Merge pull request #143 from maimairel/patch-1

Make '0' possible as a field value. Validate for empty strings instead of empty()
update version # info
update version # info
made changes to numeric slider styles
Merge branch 'wip' of github.com:valendesigns/option-tree into feature-WPML