basicAuth: allow multiple usernames and callback func

[komuw]

• allow multiple usernames and passwords
• should take a func that gets called everytime a successful login happens
  func(r http.Request){}
  People can use that func to do things like; log people in databases etc.
  The func takes a copy of request (not pointer)

See https://github.com/komuw/srs/issues/115 to guide implementation here.