Added support for an identity aware, RBAC enforcing, mutually
authenticated, web application proxy to Teleport.
* Updated services.Server to support an application servers.
* Updated services.WebSession to support application sessions.
* Added CRUD RPCs for "AppServers".
* Added CRUD RPCs for "AppSessions".
* Added RBAC support using labels for applications.
* Added JWT signer as a services.CertAuthority type.
* Added support for signing and verifying JWT tokens.
* Refactored dynamic label and heartbeat code into standalone packages.
* Added application support to web proxies and new "app_service" to
proxy mutually authenticated connections from proxy to an internal
application.
Nic Klaassen