The `constructFromObject` factory methods should be class methods
(or "static" methods), not instance methods.
With this commit, ApiClient no longer calls the model constructors
directly. Instead, it calls the new static factory method to get the
new instance. If there is no data on the top level, null is returned.
It is still possible for users to call the model constructors
directly, of course.
* Replace jQuery with SuperAgent which works in both Node.js and browser
* Use UMD pattern (returnExports.js) to make the module exporting compatible
with all major systems: AMD, Node.js (CommonJS) and browser
* Implement support of header and form parameters. Closes#1736
* Move HTTP requesting code to `ApiClient` and allow customizing options
in it, e.g. "basePath"
* Update unit tests accordingly and add some tests for `ApiClient`
