Add grpc signer

client/client.go

cmd/cashier/main.go

cmd/cashierd/main.go

cmd/cashierd/rpc.go

lib/util.go

lib/util_test.go

proto/signer.pb.go

proto/signer.proto

regen.sh

run_tests.sh

server/signer/signer.go

server/store/store.go

vendor/github.com/golang/protobuf/ptypes/any.go

vendor/github.com/golang/protobuf/ptypes/any/any.pb.go

vendor/github.com/golang/protobuf/ptypes/any/any.proto

vendor/github.com/golang/protobuf/ptypes/doc.go

vendor/github.com/golang/protobuf/ptypes/duration.go

...github.com/golang/protobuf/ptypes/duration/duration.pb.go

...github.com/golang/protobuf/ptypes/duration/duration.proto

vendor/github.com/golang/protobuf/ptypes/regen.sh