Basic auth

Sharing note to team is sometimes dangerous because it allows shell command, any scala code run.
To provide minimum security, i have implemented basic http authentication.

if there'is conf/realm.properties exists, browser asks user name and password.

conf/realm.properties is jetty realms file. one example is
```
zeppelin: zeppelin,user
```
for more information of this file, please check HashLoginService section in https://wiki.eclipse.org/Jetty/Tutorial/Realms.

One problem is, it authenticate webapp. but not websocket.

* [x] webapp authentication
* [ ] websocket authentication