Updated the QueryParamSerializer to serialize "timestamps" while observing the format.

* The default format is pulled from the service API configuration.
* The timestamp param can also pull a format from the timestamp shape.
Updated the XML builder to observice shape-specific timestamp formats.

The XML builder previously would use the service specific timestamp
format specific in the API configuration.  It will also now check
the timestamp shape for a format.
Resolved a few issues with Date params.

* All three serializers (JSON, Query and XML) accept dates
  as Date objects, strings and unix timestamps (numbers)

* All three serializers (JSON, Query and XML) now obey the
  configured timestamp formats, including:

  * iso8601
  * rfc822
  * unixTimestamp

Fixes #89
Fixes #99