Strptime date parsing as a fallback

Modified string_optimistic_date_converter to use Date.strptime as a fallback parser. This is to handle mm/dd/yyyy parsing in Ruby 1.9.3, since Date.parse no longer handles that format as of 1.9. Tests included.  Two digit year parsing is a bit of a problem still -- it works in 1.8.7 but not in 1.9. Test case handles this discrepancy, but it's clearly not ideal to have different results depending on the ruby version.