put jupyterhub version in logs and headers

jupyterhub/handlers/base.py

@@ -16,6 +16,7 @@ from tornado.ioloop import IOLoop
 from tornado.web import RequestHandler
 from tornado import gen, web
 
+from .. import __version__
 from .. import orm
 from ..objects import Server
 from ..spawner import LocalProcessSpawner
@@ -124,6 +125,7 @@ class BaseHandler(RequestHandler):
         By default sets Content-Security-Policy of frame-ancestors 'self'.
         """
         headers = self.settings.get('headers', {})
+        headers.setdefault("X-JupyterHub-Version", __version__)
         headers.setdefault("Content-Security-Policy", self.content_security_policy)
 
         for header_name, header_content in headers.items():