Allow binding to IPv6 address

[ire4ever1190]

Closes #303

Example here can be accessed at http://[::1]:8080

import asyncdispatch, jester, os, strutils

router myrouter:
  get "/":
    resp "It's alive!"

proc main() =
  let port = 8080.Port
  let settings = newSettings(port=port, bindAddr="::1")
  var jester = initJester(myrouter, settings=settings)
  jester.serve()

main()

Does this by parsing the passed bindAddr and seeing if its a IPv4 or 6 address (defaults to IPv4)