Connection getters

Adding Connection->getApiInstance(…) and Connection->getApiClass(…) for convenience.
Based on branch "renaming-class".
There is no harm in doing "new" yourself and you get exactly the same object, so go for the not documenting-way :)

I guess in a coming pull request there will be two more ways to get the instance anyway:

    ->Assortment()
       ...and...
    ->__call('Assortment')