ec2: Correctly refresh IAM credentials.

Analogous to the patch to sqs in #111.

The tests don't seem to run on my machine at present, so I haven't run them:

```
go test github.com/nelhage/goamz/ec2
# github.com/nelhage/goamz/ec2_test
ec2/ec2_test.go:824: undefined: ec2.FakeTime
ec2/ec2_test.go:825: undefined: ec2.FakeTime
ec2/sign_test.go:15: undefined: ec2.Sign
ec2/sign_test.go:28: undefined: ec2.Sign
ec2/sign_test.go:46: undefined: ec2.Sign
ec2/sign_test.go:53: undefined: ec2.Sign
ec2/sign_test.go:65: undefined: ec2.Sign
FAIL	github.com/nelhage/goamz/ec2 [build failed]
```