Resolve "Improve reload"

Closes #25.

It adds configuration reload without restarting application:

• loggers and watchers are updating on SIGHUP signal if the associated configuration is modified. Checks are launched for new watchers and cancelled for deleted ones. For updated watchers, it cancels planned checks to relaunch new ones (even if the watcher update does not directly concern the service configuration).
• watchers list is updated on JS side (watchers are now sorted alphabetically, to keep order on reload).
• it handles tags update when reloading watchers list (note: a new route is added to access directly a tag category). An error message indicates if a tag no longer exists.
• some refactoring (the Server class is moved in a dedicated file).