accounts: use pointers consistently

Account is now always a non-pointer. This will be important once
the manager starts remembering accounts.

AccountManager is now always a pointer because it contains locks
and locks cannot be copied.

accounts/accounts_test.go

@@ -3,10 +3,11 @@ package accounts
 import (
 	"testing"
 
+	"time"
+
 	"github.com/ethereum/go-ethereum/crypto"
 	"github.com/ethereum/go-ethereum/crypto/randentropy"
 	"github.com/ethereum/go-ethereum/ethutil"
-	"time"
 )
 
 func TestAccountManager(t *testing.T) {