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.
Looks good, nice work!  One question, how does getAuth switch between Windows SSPI and embedded NTLM?