Client-side Dialogs

Provides server-side method of invoking client-side dialogs, and (initially) a `LockCommand` command which implements the dialog:

![jabbr-lockcommanddialog](https://f.cloud.github.com/assets/1687386/1520448/acd7def0-4b7b-11e3-80b4-e56f1c65e786.png)