Only read NDEF messages from tags verified to have NDEF messages.

The phLibNfc_Ndef_CheckNdef() function reports the NDEF card state.
Attempting to read an NDEF message from a tag in the INVALID or
INITIALIZED state fails to call the read NDEF callback function. Don't
read NDEF messages unless the tag is in the READ_WRITE or READ_ONLY
states.
Merge pull request #132 from amccarthy/master

Only read NDEF messages from tags verified to have NDEF messages.