Set up an area for the enhanced 1.2.x interface.

* This creates a new package specifically for the 1.2 interface.
  The idea is that we could collect interface enhancements in
  similar packages as we go on.
* Deprecated the old `Sirius` (1.1.x) interface.

Discussion questions:
* What else should go into this package? Ideally this should be everything that a client would need to interact with.
* What do you think of creating a versioned package like this?
* Is the package in the right place?
* Does the package need a `package.html` (or whatever the ScalaDoc equivalent is)?
* I suspect we will find we want classes like `Sirius1Dot2Impl` that are essentially interface adapters to a single `SiriusImpl`. One complicating factor is that `SiriusImpl` is a "public" class in 1.1.x because the `SiriusFactory` returns it. So probably the single, private core class needs a different name (`SiriusFacade`?).

References:
* PRs #21 and #23 are related.
* Issues #8, #11, #17, #19, and #25.