Changed URN to URI

Followed Emilio’s suggestion (2014-2-2) to change URN fields to a more
generic, but unique and persistent URL.  This is slightly more
redundant, but much more flexible.  Emilio notes “Some unique, external
identifiers out there are actually URL's and have no equivalent URN;
eg, IOOS MMI references for variables, like this:
http://mmisw.org/ont/ioos/parameter/acidity. FYI, in such cases,
there's no "resolver", b/c the external identifier is already a
persistent URL that's fully self-documenting.:”