Add `x-pre-hashed` field during command initialisation

This patch allows clients to store the SHA-256 hash of the password in their bloostamp file.
This doesn't really inherently make it a more secure system, but allows clients to __not__ store a password, allowing user input when required (only during sending coins). This also means that users can use a password they can easily remember, perhaps saving the day.

The only drawback here is that this is still entirely vulnerable to a MITM attack, so SSL would have to be implemented.

Usage is simply as such:
```json
{
    "cmd": "send_coin",
    "to": "FOO123456....",
    "addr": "HYPOTHETICAL_BLOOCOIN_ADDRESS",
    "pwd": "sha256123aef5A....", // clearly not real
    "x-pre-hashed": null
}
```

So this coupled with SSL could make bloostamps less like a "come and get me" and more just an easy way for clients to store relevant addresses.


Discussions would be nice, I'm not 100% this is a decent addition, but in my mind it could clean up how clients interact with users. Definitely SSL would be required to really make this better.