Rolling back some earlier Pester Scope changes

Per discussions with @nohwnd, the idea of having "It" be its own Pester scope (clearing Mocks and TestDrive contents the same way Context and Describe do) feels a bit awkward in practice.  We're starting to roll back those changes until we find the right balance; for now, Mocks are still affected by It scopes, but TestDrive has reverted back to Pester 2.0 behavior (only rolling back TestDrive contents when exiting a Context or Describe block.)

Also, default behavior of Assert-MockCalled has changed to be more in line with Pester 2.0 behavior, possibly allowing old test scripts to function without any modifications.  The default value for the Scope parameter is now either Context or Describe, depending on whether there is a currently-active Context block.  You can still pass in -Scope <Describe | Context | It> to override this behavior, if desired.

Tests of these bits have been updated to match the new behavior.  Possibly more changes coming along these lines, once we've tested this revision a bit.
Merge pull request #164 from dlwyatt/ScopeBehaviorChanges

Rolling back some earlier Pester Scope changes