Error codes

[iffy]

Pull request for structured error codes to address #29. If GitHub doesn't make a nice HTML-viewable diff of the protocol, I'll put one up. The diff to look at is protocol.md. Changes can best be summarized using the AccountSet.

Before this PR:

{
  "errors": ["You must reauthenticate."],
  "accounts": [
    {
      "org": {
        "domain": "mybank.com",
        "sfin-url": "https://sfin.mybank.com"
      },
      "id": "2930002",
      "name": "Savings",
      "currency": "USD",
      "balance": "100.23",
      "available-balance": "75.23",
      "balance-date": 978366153,
      "transactions": []
    }
  ]
}

With this PR:

{
  "errlist": [
    {
      "code": "con.auth",
      "message": "Authentication required",
      "conn_id": "CON-10829309823094234",
    }
  ],
  "connections": [
    {
      "conn_id": "CON-10829309823094234",
      "name": "My Bank - Jeff",
      "org_id": "INST-982394823948230-2340923094",
      "org_name": "My Bank",
      "org_url": "https://mybank.com",
      "sfin_url": "https://sfin.mybank.com",
    }
  ],
  "accounts": [
    {
      "id": "2930002",
      "name": "Savings",
      "conn_id": "CON-10829309823094234",
      "currency": "USD",
      "balance": "100.23",
      "available-balance": "75.23",
      "balance-date": 978366153,
      "transactions": []
    }
  ]
}

During a migration period between the two versions, servers can provide both simultaneously like this:

{
  "errlist": [
    {
      "code": "con.auth",
      "message": "Authentication required",
      "conn_id": "CON-10829309823094234",
    }
  ],
  "errors": ["Authentication required"],
  "connections": [
    {
      "conn_id": "CON-10829309823094234",
      "name": "My Bank - Jeff",
      "org_id": "INST-982394823948230-2340923094",
      "org_name": "My Bank",
      "org_url": "https://mybank.com",
      "sfin_url": "https://sfin.mybank.com",
    }
  ],
  "accounts": [
    {
      "org": {
        "domain": "mybank.com",
        "sfin-url": "https://sfin.mybank.com"
      },
      "id": "2930002",
      "name": "Savings",
      "conn_id": "CON-10829309823094234",
      "currency": "USD",
      "balance": "100.23",
      "available-balance": "75.23",
      "balance-date": 978366153,
      "transactions": []
    }
  ]
}

Perhaps we should be allowed to specify a minimum version when making a request to test that things won't break for us when the old stuff is eventually removed? Something like ?min-version=2