Amended ISO8601 date formatter

Implements full ISO8601 Date support.

Changes:
- Decoding will now always take UTC offset into account (previously it was ignored) 
- Decoding now allows timestamps without colons
- Decoding allows timestamps without minutes and seconds
* The date encoder gained several config options:
  * Using hyphens between date fields (default: off)
  * Using colons between time fields (default: on)
  * Encoding to UTC instead of local time (default: local)
  * Appending UTC offset to local time (default: off)
  * Appending milliseconds (default: off)
- Default encoder config is currently set to provide 100% backward compatibility
- The DateFormatter instance is exposed as xmlrpc.dateFormatter,
  to allow for formatting option changes in implementations (see the updated [README.md](https://github.com/patricklodder/node-xmlrpc/blob/iso8601_merge/README.md))
- New unit tests for the date formatter under test/date_formatter.js