add basic CLI and config file support

See `jupyterhub -h` for common shortcuts

default config file: `jupyter_hub_config.py`
generate config file with: `jupyterhub --generate-config`
non-default config file: `jupyterhub -f myconfig.py`

jupyterhub/tests/test_app.py

@@ -0,0 +1,11 @@
+"""Test the JupyterHubApp entry point"""
+
+import sys
+from subprocess import check_output
+
+def test_help_all():
+    out = check_output([sys.executable, '-m', 'jupyterhub', '--help-all']).decode('utf8', 'replace')
+    assert u'--ip' in out
+    assert u'--JupyterHubApp.ip' in out
+
+