Move SiriusFactory to top-level api package.

This is a re-spin of #27 but also incorporates the intent of #68.
This should basically mean that a client of the Sirius library
ought only to directly use classes, objects, and traits from the
`com.comcast.xfinity.sirius.api` package, and that, going
forward, those are the files that must retain backwards
compatibility.

The main changes involved are:

* Additional extension methods for 1.3 are now collected in the
  `Sirius1Dot3` trait that extends the existing `Sirius` trait.

* Existing `SiriusFactory` was hoisted up to `api` package and now
  returns a `Sirius1Dot3` instead of a `SiriusImpl`. Clients ought
  to be depending on a trait rather than a concrete class here.

* There is still a `SiriusFactory` in the old location, but it
  just delegates to the one in the new location, then downcasts
  the return result to a `SiriusImpl`. This `SiriusFactory` is
  marked as deprecated.