Update UnidataDD2MI.xsl

Integration of NGDC modifications. These have been tested on IOOS metadata since July of this year. 
This is a summary: 
*2014-11-25. fixed error that was outputting '::' at the end of a date.*
*2014-07-24. test for null date and fixed another seconds since.... input. Translates UTC dates to ISO dateTime and 8601 basic dates to 8601 extended.*
*2014-07-17. updated dimensionResolution to handle many but not all ISO 8601 durations*
*2014-07-16. updated uom units for time to display only seconds or days when includes "days since..." or "seconds since..." text. *
* 2014-04-19. Changed order to look in CFMetadata group first in *unit variables. Added normalize-space dimensionResolution param to fix vertical units error. *
* 2012-05-11. 2.3 (slight revision): Changed the attribute name from coverage_type to coverage_content_type to be more correct and directly comparable to ISO....
Merge pull request #3 from amilan17/NGDC-Updates

Merge NGDC and GitHub versions UnidataDD2MI.xsl