RunInstances: specify block device mappings

Having some trouble running tests:

```
$ go test
warning: building out-of-date packages:
	github.com/mitchellh/goamz/ec2/ec2test
	github.com/mitchellh/goamz/testutil
installing these packages with 'go test -i' will speed future tests.

# _/Users/rgarcia/github/goamz/ec2_test
./ec2_test.go:97: unknown ec2.RunInstances field 'BlockDeviceMappings' in struct literal
./ec2_test.go:729: undefined: ec2.FakeTime
./ec2_test.go:730: undefined: ec2.FakeTime
./ec2_test.go:730: defer requires function call, not conversion
./sign_test.go:15: undefined: ec2.Sign
./sign_test.go:28: undefined: ec2.Sign
./sign_test.go:46: undefined: ec2.Sign
./sign_test.go:53: undefined: ec2.Sign
./sign_test.go:65: undefined: ec2.Sign
FAIL	_/Users/rgarcia/github/goamz/ec2 [build failed]
```