Support for proxy servers requiring authentication

This is intended to fix the shortcomings of the library when used behind proxy server requiring authentication (supports authentication using HTTP BASIC and DIGEST methods).

The integration test included does test proxy connections, however an external proxy server (e.g. Squid) needs to be configured to test the proxy authentication as the "InJVM"-proxy server (Jetty Servlet) does not support it (maybe consider to replace with LittleProxy).