Previously, running touch config.json would result in the server

reloading, but if the configuratino file was overwritten (say by a text

editor), rb-gateway would crash. Now we have a ConfigWatcher

abstraction that wraps fsnotify to (1) wait for the file to be written

and (2) re-watch the file. It also provides a utility to force reload

the configuration (which is triggered by SIGHUP currently).

Ran unit tests.
Ran touch config.json and saw the config reloaded.
Edited config.json and saw the config reloaded (and rb-gateway did

  not crash).
kill -sHUP (pgrep rb-gateway) and saw the config reloaded.