Windows mutex seems to be behaving correctly according to unit tests. Firs safe place to commit

Change-Id: I271674b421649c869d78908dd3935061887b84ab
Reviewed-on: http://www.matthewh.me/gerrit/49
Reviewed-by: Matthew Hoggan <mehoggan@gmail.com>
Tested-by: Matthew Hoggan <mehoggan@gmail.com>