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.
sspi is included only when building for windows and ntlm is included only when building not for windows.
ntlm.go has the build constraint // +build !windows
and sspi is named sspi_windows.go with a build constraint in its name.
For build constraints documentation see: http://golang.org/pkg/go/build/