Nested stubbing doesn't work

[searls]

Right now, testdouble.js blows up if—in the same statement as a stubbing—the user attempts to stub a different test double.

Example failing test:

  describe 'nested whens', ->
    Given -> @knob = td.function()
    Given -> @door = td.function()
    Given -> td.when(@knob('twist')).thenReturn
      door: td.when(@door('push')).thenReturn('open')
    When -> @result = @knob('twist').door('push')
    Then -> @result == 'open'

This is because the call store has just a single lastCall that it tracks, when in reality it should probably resemble a stack instead.