ServerIdentity.ID shouldn't rely on `String`

[ineiti]

In https://github.com/dedis/onet/blob/master/network/struct.go#L175, the ID of ServerIdentity depends on the output of Point.String(), which is bad, because String() is a method for users and is supposed to change.

Fix:

1. easy: use MarshalBinary and hex.EncodeToString
2. better: add a RfcMarshaller that is semantically stronger than a MarshalBinary

IMO, 1. is enough, as for me MarshalBinary shouldn't change. But see discussion in #405