Add initial decryption of the double encryption and T3 signing

* Add first RSA decryption of the encrypted event key
  (with giver/taker key) to get AES key for further decryption.
* Add AES decryption code using Crypto-JS, it runs and produces a
  plaintext, but is it not the expected one.
* Add base64 encoding of the oracle RSA key to avoid PEM newline
  issues in JS.
* Change RSA encryption padding from PKCS#1 to OAEP.