[patch] intake DateString -> Date

[Stephen Haberman <st...@chase3000.com>]

I posted a patch for the DTD the other day changing Date to DateString
and removing DateKey to match the Fulcrum FieldFactory code. However,
I'm assuming the Turbine 2.x uses Date instead of DateString and Date
also makes more sense than DateString as it's more consistent with the
others...e.g. boolean/BooleanField, int/IntegerField. 

Date/DateField would be ideal, so if the DateStringField class could
also be renamed to just DateField, I think it makes more sense.

But besides that, here's a patch for FieldFactory to return the
DateStringField for type="Date" instead of type="DateString".

The current patch ignores "DateString" just as the existing version
ignores "Date", however, I think this is appropriate as "DateString" is
not in the intake.dtd.

- Stephen