* fix java doc for okhttp-gson
* add ci test for javadoc
* fix return type for java doc
* fix warning/error with javadoc
* fix error/warning in javadoc
* fix 2 warnings related to docstring
* remove trailing space for okhttp tmeplate
- Rename ApiInvoker to ApiClient
- Make ApiClient pluggable by allowing setting the ApiClient field of
API classes
- Introduce a Configuration class, containing the default
ApiClient (which is also customizable)
- Move basePath from API class to ApiClient
- Change static methods in ApiClient to instance level