(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)



If this is mounted at a prefix, like `/puppet` or whatev, then it is appropriate to have this `not-found` handler here.  The reason we couldn't use it before is because both ring handlers had to be mounted at `/`.