Separation of depositor and ticket signer

We should allow the account that deposits ETH to the PM contract to be different than the account that signs tickets. A depositor could specify account(s) that are allowed to sign on its behalf. See https://github.com/livepeer/LIPs/issues/13 for some benefits of this approach.

A rough sketch of what the implementation could look like:

struct Sender {
    ...
    mapping (address => bool) signers;
}

struct Ticket {
    ...
    address sender;
}

function isValidSenderSig(address _sender, bytes _sig, bytes32 _ticketHash) internal view returns (bool) {
    // Recover signer address from _sig and _ticketHash
    return senders[_sender].signers[signer];
}

[dob]

One other idea that isn’t fully thought through is “allowances” per signer key - much like the approve() method on ERC-20. The central key that manages funds could manage allowances independently of being online on many ingest machines to sign PM tickets.

[yondonfu]

Closed by #265