NEW: Pass a whole new object containing required properties to postJSON and getJSON functions instead of a reference to Client#options.

This allow properties to be static across the whole request/response cycle, particularly interesting for the property bigint which is quite subject to change between requests and since bigint value is used to determine JSON (de)serializer so at request time and response time you see where I am going!
NEW: Add migration guide between 0.3.x and 0.4.x.