IonPopup - return a handle to close individual popups + bug fixes with multiple IonPopups

[ClarenceL]

• IonicPopup - add cssClass option which is supported in official Ionic Framework
• IonicPopup - return a handle with a close function, to allow closing individual IonPopups
• IonicPopup - fixed a bug on IonPopup.buttonClicked, not referencing the correct popup in cases where there are multiple popups

[ClarenceL]

Now you can nest IonPopups within each other, and close individual popups

# TODO: form to submit email
    IonPopup.show({
      title: 'Forgotten Password'
      template: ...
      cssClass: 'popup-wide popup-forgot-password'
      buttons: [{
        text: 'Cancel'
        type: 'button-default'
        onTap: ( ev, t) ->
          return true;
      },
      {
        text: 'Submit'
        type: 'button-positive'
        onTap: ( ev, t ) ->

          resetEmail = $('#forgot_password_container .forgotten-password-email').val()

          unless resetEmail?.length
            subPopup = IonPopup.show(
              template: 'You must enter an email'
              buttons: [{
                text: 'Ok'
                type: 'button-default'
                onTap: (ev) ->
                  subPopup.close()
              }]
            )