Good defaults for cells with no values

FXForms will crash if you give it the wrong value for a cell type. Because of that, we have to be a little smarter about what we give each cell for a default.

Ref: #13, #10.
Comparing `NSDate.date == NSDate.date` doesn't work, because it recognizes the nanosecond between them. But the `to_s` for each is close enough.