(SERVER-113) admin http endpoint

This commit adds a new TK service and HTTP endpoint for puppet
admin functionality.  For now, it just has a placeholder
"/hello" route, but the scaffolding is in place for wiring
in the real functionality, including:
    
* Support for client whitelist configuration
* Versioned routes (just /v1 for now)



Should we do our own `not-found` route or just let Ring's default one be used?  Seems like we put it in once and took it back out again for the CA or master service -- although that might have been because of the root-level route conflict between the two, can't remember...