Renamed option --debug to --show-backtrace
For ruby >= 2.0.0 headers must be an hash

This should give us support for ruby >= 2.0.0 this was the only problem
that I was able to find
Bind userinfo with Authorization header

A change in userinfo should be reflected in Authorization header, a
change in Authorization header should be refleted in userinfo.

This was not easy because of the complex dependecies between requests's
methods, in the end I resolved with an hack to avoid an infinite
recursion when build_uri (a change in Authorization header will change
userinfo that will change the uri that needs to be rebuilded)

A major refactoring is needed to keep the request habitable