Checkpoint : MockContext.Recorder works

* Big idea: MockContextAttribute marks the beginning of a
record/playback session
This feature is based on the Xunit.BeforeAfterAttribute class.
* MockContext renamed as MockContextRetired
* FakeContext renamed as MockContext
* Removed last remnants of Reactive from tree
Disabled one example and two tests as a result. I've got TODOs for
bringing these examples/tests back.
Checkpoint: MockContext.Player works