Receiving NDEF Messages

Rather than polling for NFC messages, I think it's helpful to think of the
browser as receiving NFC events.

Proposal - replace ontagfound and onpeerfound with onmessage

It should not matter if the messages we receive is from a NFC tag or a NFC
peer.  (On platforms like Windows Phone 8 we can't tell if the message is
from a tag or a peer.) The operating system reads the NDEF message and
fires an event. The NFC API should provide an onmessage handler.

    nfc.onmessage = function(nfcEvent) { ... };

The NfcEvent should include an message property which is an array of
NDEFRecords.

The NfcEvent may include a tag property which is an NfcTag object that can
contain data such tag id, nfc forum type, tag technology, max size,
read-only status, etc.

NfcEvent should replace NFCTagEvent and NFCPeerEvent

NFCTagEvent, NFCPeerEvent and NDEFMessageEvent should be removed.

NDEFMessage should be removed because an NDEFMessage is just an array of
NDEFRecords.