dreadnot web should watch for changes to htpasswd

This introduces a few changes:

  * getUsers now reads from htpasswd asynchronously
  * loadDBFromFile calls an fs.watch, which always sends SIGHUP on change
  * loadDBFromFile uses bind to pass the file name along to onSIGHUP
  * onSIGHUP handles the reloading of the files
clean up htpasswd file watching
Merge pull request #56 from racker/jordane/watch_htpasswd_file

dreadnot web should watch for changes to htpasswd