Make api url configurable somehow

https://github.com/UniversityRadioYork/myradio-go/blob/master/api.go#L20

``` go
func NewSession(apikey string) (*Session, error) {
    url, err := url.Parse(`https://ury.york.ac.uk/api/v2`)
    if err != nil {
        return nil, err
    }
...
```
