New Feature: Support 401 challenges for Basic authorization

[GoogleCodeExporter]

Issue 46 gave us pre-preemptive basic authorization via username:password in 
the URL, this is issue is for allowing us to respond to 401 challenges from 
servers (i.e. client sends request -> server sends 401 -> we prompt user for 
username and password and then resend request to server with Authorization 
header).

Original issue reported on code.google.com by stephen....@gmail.com on 7 Sep 2014 at 2:59

[GoogleCodeExporter]

The challenge could have checkbox (checked by default) that indicates that the 
proper Authorization header should be inserted into the request headers for 
future pre-emptive authorization

Original comment by stephen....@gmail.com on 8 Oct 2014 at 4:18