This patch does a few things to simplify the public key classes
(RSA, DSA and ElGamal):
* It removes the Crypto.PublicKey.pubkey module. The 3 classes
do not have an ancestor anymore.
* Methods sign(), verify(), encrypt(), and decrypt() are removed.
* Methods blind() and unblind() are removed.
* Methods can_sign() and can_encrypt() are removed.
* The 3 classes cannot be pickled anymore.
When the various components are assembled into an RSA,
DSA or ElGamal key via the construct() method, we must verify
as much as possible if the result is indeed a valid key.
This will avoid the previous situation where scripts like the old "test.py"
get included accidentally in a release. It also frees us to put additional
build scripts in the top-level directory of the source tree.
