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
}
Document create_case options in the README
Fix retaining memoizer

`reduce` uses the return value so we must always return the api_hash.
Merge pull request #4 from joeljunstrom/create-case-options

CaseSerializers adds options to the request body