Add support for an optional, secure gRPC service

[jshlbrd]

This PR adds a gRPC service to Stenographer and implements a call to retrieve PCAP via stenoread. The gRPC service is designed to only use a secure channel (due to the sensitivity of packet data) with SSL certificates that are managed separately from those created by stenokeys.sh (due to users needing to manage remote access to many servers running Stenographer, though technically users could share the certs used by the local Stenographer HTTP service). This commit explicitly does not address any client-side integration with the gRPC service, but clients can build interaction based on the protobuf and documentation in DESIGN.md. Copyrights have been added per CONTRIBUTING.md and the CLA signed. Below is a summary of the additions and changes:

• Added rpc/rpc.go
• Added protobuf/steno.proto
• Changed config/config.go to support Rpc configuration
• Changed stenographer.go to check for Rpc configuration and (if found) run gRPC server as a goroutine
• Updated DESIGN docs to describe Rpc functionality

[gconnell]

This looks great, merging!

Note that Travis is dead, so the failure there is unrelated to this.