Windows Authentication and Single-Sign-On

@denisenkom please review and test if you can

Windows authentication and single-sign-on is implemented using the windows sspi api without cgo. The implementation is in sspi_windows.go and can use kerberos or ntlm to authenticate. I have tested the code only on 32-bit windows machines; needs testing in 64-bit Windows.

ntlm v1 windows authentication is implemented in ntlm.go for non windows machines. A dependency to md4 is introduced. Might not work on newer windows servers that are configured to accept only ntlm v2.