Kerberos authorization support

[mrogala-sauce]

Support for using Kerberos SPNEGO headers to be injected into proxied connections.

How this works:

• user configures Kerberos KDC server connection (using regular krb5.conf file and usertab)
• user configures a list of web hosts that are known to require Kerberos SPNEGO authorization header
• for each forwarded connection, kerberos adapter fetches Kerberos ticket for particular host (if needed), converts to SPNEGO format and injects into request header (tickets are cached internally).
• both host and upstream proxy Kerberos auth is supported (using either Authorization or Proxy-Authorization header).

[markamsauce]

Looks good. One minor comment for doc but can be ignored if no other changes planned to this PR.

[markamsauce]

Maybe it's better to clean examples from any "private names" and put root@host:/etc/krb5kdc# ktutil or jus root:/etc/krb5kdc # ktutil.

[mrogala-sauce]

good point, changed