lack of support for generation of `CKK_GENERIC_SECRET` key type.

[keldonin]

The library does not support creation of keys of type CKK_GENERIC_SECRET.

• there is no default mechanism set for KeyType.GENERIC_SECRET, it should be Mechanism.GENERIC_SECRET_KEY_GEN
• the library does not handle properly CKA_VALUE_LEN attribute: it is only set for AES keys. It result in a CKR_TEMPLATE_INCOMPLETE error for other key types (exept those where a key value length is not needed, such as DES).

The first problem is illustrated by the following excerpt:

Traceback (most recent call last):
  File "pkcs11/_pkcs11.pyx", line 98, in pkcs11._pkcs11.MechanismWithParam.__init__
KeyError: <KeyType.GENERIC_SECRET>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/home/eric/stage/./test.py", line 117, in <module>
    generate_p11perftest_keys(args.library, args.slotindex, args.password, not args.noop)
  File "/usr/home/eric/stage/./test.py", line 27, in generate_p11perftest_keys
    des1 = session.generate_key( KeyType.GENERIC_SECRET,
  File "pkcs11/_pkcs11.pyx", line 382, in pkcs11._pkcs11.Session.generate_key
  File "pkcs11/_pkcs11.pyx", line 100, in pkcs11._pkcs11.MechanismWithParam.__init__
pkcs11.exceptions.ArgumentsBad: No default mechanism for this key type. Please specify `mechanism`.

the second problem is illustrated by the following excerpt (once a mechanism has been set explicitely to clear the first issue):

Traceback (most recent call last):
  File "/usr/home/eric/stage/./test.py", line 117, in <module>
    generate_p11perftest_keys(args.library, args.slotindex, args.password, not args.noop)
  File "/usr/home/eric/stage/./test.py", line 27, in generate_p11perftest_keys
    des1 = session.generate_key( KeyType.GENERIC_SECRET,
  File "stringsource", line 415, in View.MemoryView.memoryview.__getitem__
  File "pkcs11/_errors.pyx", line 88, in pkcs11._pkcs11.assertRV
pkcs11.exceptions.TemplateIncomplete

[MatthiasValvekens]

Since #141 was (finally) merged now, this is fixed ;).