Simplify the http-client library

[cgay]

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.

[fracek]

LGTM.