Provide a Signer type with more convenient methods

It's common to use one set of keys for all requests, so put the keys
into the receiver. Possible interface:

``` go
type Signer struct {
    Keys

    Service *Service // if nil, DefaultService is used

    Time func() time.Time // if nil, time.Now is used
}

func (s *Signer) Sign(*http.Request) error
```

With addition of an underlying http transport, this type could also
satisfy #12.
