Fixes #7 :
Add api module which provides only API (no JAXRS import)
All builders have interfaces (Objects are built in core module based on a given contract)

So we've cut down clear model between API and implementation

API is looking for implementation by using ServiceLoader (JavaSE) and in OSGi context there is an activator on the implementation that register the implementation.
If multiple implementation are provided, only the first one is used.