date/time formats

[mbaudis]

I suggest specification of ISO8601 for date/time values (see GA4GH; https://github.com/ga4gh/schemas/blob/master/doc/source/api/metadata.rst).

This requires conversion of the current examples:
"created": "!!date 2016 1 14" => "created": "2016-01-14"

... and replacement of the dtjson.py code.

[cmungall]

Currently we support both yaml and json. As json is the canonical form, json is mostly a subset of yaml, this generally works fine... except for dates. I'll look into this some more