Added DateColumn class
Added specs for DateColumn
Expect StandardError in rspecs
DateColumn passes all specs
Use the new DateColumn in the csv parser
Whitespace fixes
Added some more tests for date
Use date format as before
Added a comment and clarified specs
Removed old comments and corrected typo
Merge pull request #35 from BlackEdder/date_format

Date format

Raise error if date format cannot be determined automagically. Fixes bug #18