CaseSerializers adds options to the request body

Options is passed as the second argument to `Egree::Client#create_case`

Supported options for now is:

`postback_url` – The url egree should send a payload to when the case is signed.

`continue` – A hash of options for where the user should end up.

```
continue: {
  name: "Back to the site",
  url: "http://example.com/thanks",
  auto: true
}
```

Closes #1