Add dns.tsig.Key class.

If dns.tsigkeyring.from_text() creates dns.tsig.Key objects with the
default algorithm, that causes problems for code that specifies a
different algorithm.  There's no good way to handle this, so change
dns.tsigkeyring.from_text() to not create dns.tsig.Key objects unless it
knows the algorithm.

This creates a new class to represent a TSIG key, containing name,
secret, and algorithm.

The keyring format is changed to be {name : key}, and the methods in
dns.tsigkeyring are updated to deal with old and new formats.

The Message class is updated to use dns.tsig.Key, although (to avoid
breaking existing code), it stores them in the keyring field.

Message.use_tsig() can accept either explicit keys, or keyrings; it will
extract and/or create a key.

dns.message.from_wire() can accept either a key or a keyring in the
keyring parameter.  If passed a key, it will now raise if the TSIG
record in the message was signed with a different key.  If passed a
keyring containing keys (as opposed to bare secrets), it will check that
the TSIG record's algorithm matches that of the key.