Implement BIP38 password encrypted private keys, decryption only. 

FIRST TAKE, REQUEST FOR REVIEW

Open questions:
- We have two Scrypt impls available -- which one should we prefer?
- ECKey constructor used is deprecated, is there a better one? We need to pass private key & compressed flag.
- Dedicated exception for when passphrase was (most likely) entered wrong?
- I'd like to replace Cipher.getInstance("AES/ECB/NoPadding") by manually instantiating and chaining the concrete impls.
This should probably go in the crypto package.