Merge pull request #1711 from Gustav-Simonsson/timestamp_big_int

Add tests for uncle timestamps and refactor timestamp type
(cherry picked from commit abce09954b)

core/vm/vm.go

@@ -491,7 +491,7 @@ func (self *Vm) Run(context *Context, input []byte) (ret []byte, err error) {
 		case TIMESTAMP:
 			time := self.env.Time()
 
-			stack.push(new(big.Int).SetUint64(time))
+			stack.push(new(big.Int).Set(time))
 
 		case NUMBER:
 			number := self.env.BlockNumber()