New function for private key package

[kastolars]

• [x] Add Bytes []byte and Hex string fields to AurumPrivateKey struct
• [x] function called New, which creates an AurumPrivateKey struct
• [x] Key field is populated by generating a random private key (look here for to see how this is done)
• [x] Bytes field set to the return of privateKey.Encode with the Key field as an argument
• [x] Hex field set to the return of hex.EncodeToString with Bytes field as argument
• [x] Test should just make sure that Bytes field is the encoded version of Key and Hex field is hex-encoded version of Bytes field

[HarryL5004]

Completed