Generalize auth/types.StdSignature (cosmos#4507)

New Signature interface available in the top level types package.
auth.StdSignature implements such interface. User defined auth
module can now define their own custom signature types.

Work carried out in the context of the following issues:
- cosmos#4488
- cosmos#4487

test_util.go

@@ -5,10 +5,11 @@ import (
 
 	"github.com/tendermint/tendermint/libs/log"
 
+	dbm "github.com/tendermint/tendermint/libs/db"
+
 	bam "github.com/cosmos/cosmos-sdk/baseapp"
 	sdk "github.com/cosmos/cosmos-sdk/types"
 	"github.com/cosmos/cosmos-sdk/x/staking"
-	dbm "github.com/tendermint/tendermint/libs/db"
 )
 
 // NewSimAppUNSAFE is used for debugging purposes only.