Unit test for foreign id

this reproduces bug #37 in a simple test
hm, `Value` must be lowercase here or it is ignored. but the error later on is

```
Expected error:
        <*errors.errorString | 0xc2100f8eb0>: {
            s: "expected struct to have field Foreign",
        }
        expected struct to have field Foreign
    not to have occurred
    /home/travis/gopath/src/github.com/univedo/api2go/integration_test.go:45
```

which is correct, because it is not enough to have an ID field as a struct, you also need another struct which points to `Foreign`. Change it and see if the test then works, I bet it will.