Some meaningless syntax changes.

Use curlies for one-line "if" blocks.
Improve handling of authenticators.

Now the special case is for Access-Request, and everything else behaves
as Accounting did previously. Access-Request's authenticator is random,
and to the best of my knowledge all other Request types just use
the normal checksum authenticator (since there is no password), so when
decoded "Request" types (other than "Access-Request") validate the
authenticator accordingly. Similarly, when encoding packets, only use
the create the random authenticator for "Access-Requests", and for
everything else calculate the regular checksum (if you use prepare_response,
the checksum will properly be over the request packet's authenticator).
Add radius.verify_response method.

You can use this to verify a response packet. For example, if you
send an Accounting-Request, after receiving the Accounting-Response
you should use verify_response to make sure the response's
authenticator is correct (i.e. the other side knows the shared secret).