`diffsync.DiffSync` naming

[Kircheneer]

Since we are calling subclasses of diffsync.DiffSyncadapters in the examples and even in the docs

When you initialize the Diffsync Adapter class

maybe we should just rename the class to diffsync.Adapter? To me this is clearer.

[glennmatthews]

I'd be in favor of this change. :-)

[chadell]

It looks good to me too

[Kircheneer]

We should keep the old name around for at least a couple of minor versions I think, but change the docs to point to the new one.

[Kircheneer]

Closed with #242