Change transfer_object to transfer_coin in Client API

[lxfind]

As described in #612 , we only support native transfer of the coin objects, and we don't intend to support native transfer of arbitrary objects in the near term (maybe until we figure out how to use capability to express transferability of Move struct types).
This PR updates the Client API to reflect this restriction.
Also cleaned up the checks on the authority side for code reuse.

Do we also need to make any document changes?

[gdanezis]

Quick question: why are we being special about this? Why not also handle coin transfers through the move transfer path? And then optimize this? We could get rid of quite some code no? (And I love deleting code).

[lxfind]

We totally could. If we don't have a native transfer today then we probably would hold off optimizing it until we see data. But since we have it today, I would also hold off de-optimizing it until we have data.
Intuitively it makes sense to me to have a significantly cheaper way for coin (gas included) transfers as it is likely to be common enough. But I am happy to delete latter once we are sure we don't need to add it back.

[huitseeker]

LGTM! Make sense to keep this clean (until we delete it).

Do we also need to make any document changes?

I have not noticed any.