APIClient stub

[joshwget]

Having an empty implementation of APIClient (and possibly some of the other engine-api interfaces as well) is very useful for testing. @vdemeester wrote an implementation that we use in libcompose. This might be something to consider adding to engine-api.

[vdemeester]

Note that it's the same for swarm, and several libraries that use engine-api (including mine 🐹), I also think it would make sence to have somewhere a "reference" 👼

/cc @calavera