Define the same NFCMessage both for push and read

[zolkis]

1. Currently pushMessage() takes as argument a sequence of records, whereas when NFC content is read, it is presented as an NFCMessage with a url and a sequence of records.
2. Currently developers cannot control what path is written in the Web NFC Id. The steps currently say that URL path of the browsing context is written as Web NFC Id.

It would be good if we'd have the same NFCMessage definition both for push and reads.
Also, it would be nice if developers could control the path part of the Web NFC Id, but could not fake the scheme/host/port.

These two things could be solved together by modifying the pushMessage signature to:

Promise<void> push(NFCMessage message, NFCPushOptions options);

where message.url is supposed to provide a developer defined path.
If we don't want to support this, we could also say message.url is not currently used.

[zolkis]

Fixed by #88.