Export SignedData in cms. Fixes #89

[florentchauveau]

As suggested in #89, the struct is changed to

type SignedData struct {
  *protocol.SignedData
}

The tests are updated accordingly.

[quite]

Things seem to have been moving a bit (relatively) recently, perhaps rebase this? I see the need for this, to be able to actually do some inspection of for example what one gets from ParseSignedData().