Simplify the http-client library

This does a couple of main things...

* Greatly simplify the http-client code by
  + Removing the ``<http-request>`` class and conversions from that to ``<base-http-request>``.
  + Reordering the required arguments to http-request so that the various convenience functions (http-{get,post,...}) can be defined in terms of a simple curry statement.

* Add some rudimentary documentation for http-client.  The user level doc is mainly just examples.  The ref doc is little more than a skeleton so far.
Done.