Switch from date() to DateTime() to support dates prior to 1901

[adrianbj]

https://processwire.com/talk/topic/11845-date-time-field-not-showing/

Not sure if there are any advantages to using date() at all anymore, but at least for the datetime field I think it should start using DateTime() so users can enter dates before 1901.

This is copied over from: ryancramerdesign/ProcessWire#1624

[adrianbj]

From what I understand, this change would also support dates 2038 and later: http://stackoverflow.com/a/5319762/1524576

The issue with 2038 just came up in the forums:
https://processwire.com/talk/topic/15715-cant-save-date-20-years-in-the-futre/