Replace Chad Okere's RFC 3339 parsing code by my own ...

... which (hopefully) is less broken.  Major differences are:
* 'Z' time zone is treated as UTC instead of local time zone
* fractional seconds are not parsed instead of parsing them wrongly
* pure dates, without time information, are accepted as well because that is
  what vegguide.org sends for closed_date
Add JUnit tests for Rfc3339