title: admob swap between banner & interstitial
body: <p>how can you swap between a admob banner and interstitial?
  if i load the banner on my first scene and hide it when i go to my next scene and want to load a interstitial ad it shows me a banner. and if i don't  load the banner and only the "interstitial ad scene" it loads the interstitial ad </p>
  
  <p>Banner scene:</p>
  
  <pre><code>local provider = "admob"
  local appID = "ca-app-pub-****"
  local ads = require "ads"
  
  local screenGroup = self.view
  local statusText = display.newText( "", 0, 0, native.systemFontBold, 22 )
  statusText:setTextColor( 255 )
  statusText:setReferencePoint( display.CenterReferencePoint )
  statusText.x, statusText.y = display.contentWidth * 0.5, 160
  
  local showAd
  local function adListener( event )
      local msg = event.response
      print("Message received from the ads library: ", msg)
      if event.isError then
          statusText:setTextColor( 255, 0, 0 )
          statusText.text = "Error Loading Ad"
          statusText.x = display.contentWidth * 0.5
          local screenGroup = self.view
          showAd( "banner" )
      else
      end
  end
  
  if appID then
      ads.init( provider, appID, adListener )
  end
  
  local sysModel = system.getInfo("model")
  local sysEnv = system.getInfo("environment")
  
  local screenGroup = self.view
  showAd = function( adType )
      local screenGroup = self.view
      local adX, adY = display.screenOriginX, 400
      statusText.text = ""
      ads.show( adType, { x=adX, y=adY } )
  end
  
  if sysEnv == "simulator" then
  else
  local screenGroup = self.view
      ads.show( "banner", { x=display.screenOriginX, y=400 } )
  end
  </code></pre>
  
  <p>interstitial scene:</p>
  
  <pre><code>local provider = "admob"
  local appID = "ca-app-pub-***"
  local ads = require "ads"
  
  local screenGroup = self.view
  local statusText = display.newText( "", 0, 0, native.systemFontBold, 22 )
  statusText:setTextColor( 255 )
  statusText:setReferencePoint( display.CenterReferencePoint )
  statusText.x, statusText.y = display.contentWidth * 0.5, 160
  
  local showAd
  
  -- Set up ad listener.
  local function adListener( event )
      local msg = event.response
      print("Message received from the ads library: ", msg)
      if event.isError then
          statusText:setTextColor( 255, 0, 0 )
          statusText.text = "Error Loading Ad"
          statusText.x = display.contentWidth * 0.5
          local screenGroup = self.view
          showAd( "banner" )
      else
      end
  end
  
  if appID then
      ads.init( provider, appID, adListener )
  end
  
  showAd = function( adType )
      local screenGroup = self.view
      local adX, adY = display.screenOriginX, display.screenOriginY
      statusText.text = ""
      ads.show( adType, { x=adX, y=adY } )
  end
  
  -- if on simulator, let user know they must build for device
  if sysEnv == "simulator" then
  
  else
  local screenGroup = self.view
      ads.show( "interstitial", { x=0, y=0 } )
  end
  </code></pre>