[client] Reduce complexity. Having an <http-request> class and
conversions from that to <base-http-request> just complicates the
code. Also, by changing
  http-request(url, request-method, ...)
to
  http-request(request-method, url, ...)
we can turn ~100 LOC into 5 simple curry statements.
[client] Clean up comments and add TODOs.
[client] Export http-request function
[client] Add some doc.
[client] Send / in request line for empty URI path.  Fixes #2
Merge pull request #72 from cgay/simple-client

Simplify the http-client library