Adding support for using delegated GSSCredential for Kerberos authentication
Contributed by Vipul Mehta <vipulmehta.1989@gmail.com>

Internally httpclient relies on GSS API which uses JAAS login
configuration specified by user to get the GSSCredential. This patch
will allow a user to avoid the config file and directly set a delegated
or normal GSSCredential. A normal GSSCredential can be  obtained
programatically from
spn-keytab or user-password using custom Login module.

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpclient/trunk@1623311 13f79535-47bb-0310-9956-ffa450edef68