Create Determinator#to_s and alias it to #to_liquid

Might fix #25
Merge pull request #26 from parkr/patch-1

Create Determinator#to_s and alias it to #to_liquid
When passing Determinator to Liquid, give a Time.

This means the formatting does *not* happen in `#to_liquid`.
Add tests for #to_liquid
Rename some stuff for easier understanding.