Fix Makefile build command; Define Signature interface

Makefile

@@ -28,7 +28,7 @@ build_race:
 
 test: build
 	-rm -rf ~/.tendermint_test_bak
-	mv ~/.tendermint_test ~/.tendermint_test_bak
+	-mv ~/.tendermint_test ~/.tendermint_test_bak
 	go test github.com/tendermint/tendermint/...
 
 draw_deps:

account/signature.go

@@ -9,6 +9,8 @@ import (
 
 // Signature is a part of Txs and consensus Votes.
 type Signature interface {
+	IsZero() bool
+	String() string
 }
 
 // Types of Signature implementations