Fixes, improvements, dedicated exception types and more tests

[voetsjoeba]

• Update to javaobj-py3 0.1.4 to eliminate warning about remaining data when parsing serialized SealedObjects in SecretKey entries
• Expose access to PKCS#8 encoded PrivateKey and algorithm OID in new pkey_pkcs8 resp. algorithm_oid attributes
• Explicitly supply ASN.1 specs to pyasn1 for better data structure clarity
• Adds dedicated exception types for encountering unexpected data
• Adds test case for JCEKS PrivateKey entries (RSA-1024)
• Adds test cases for unexpected Java types at or inside the SealedObject level for SecretKey entries