This changes the signature for `contact::Address::new`,
`contact::ContactCreate::new` and the type of fields
`contact::InfoData::voice` and `contact::ContactCreateRequest::voice`.
Fixes issue #13.
Ideally we might have modeled this with an enum with one named
field in the variants, but instant-xml is not yet up to the task;
in the mean-time, this provides more robust types, and manual
ToXml implementations aren't too verbose anyway.
From RFC 5731:
This element MUST contain an "avail" attribute whose value indicates
object availability (can it be provisioned or not) at the moment the
<check> command was completed. A value of "1" or "true" means that
the object can be provisioned. A value of "0" or "false" means that
the object can not be provisioned.
