Review: use httpclient gem to handle http traffic

.. and delete our somewhat flakey custom http auth and cookie code.

I chose httpclient gem because it looks decent and unlike other also good-looking alternatives (restclient for example) it handles digest auth out of the box.

Other httpclient features we'd use

* http proxy out of the box (env var http_proxy or HTTP_PROXY)
* persistent on-disk cookie store

Current state: it works, I tested 5, 6 RETS servers I could put my hands on
Todo's: more code cleanup, more test cleanup, add tests

I was prompted to work on this becase the new MLS we want to integrate with requires Basic authentication and doesn't work with  Digest authentication. Basic auth surprisingly doesn't work in the master version of rets gem.