Properly handle image heights with fade transtion

When using the fade transition with a slider that has images of varying heights, the stage height is set to the height of the tallest image. Images that are shorter than the tallest image are displayed from the upper left part of the stage. This leads to extra white space below (and potentially to the side of) shorter images. The height of the stage is always the same size as that of the tallest image and never resized to that of the current image. This problem is particularly noticeable in iOS devices in portrait mode.

As an example, below is how an shorter image is displayed in a gallery that includes taller images on iPad 3 (iOS 6):

![Screen Shot 2013-03-21 at 10 26 48 AM](https://f.cloud.github.com/assets/921795/287033/e16ddb4e-924c-11e2-86d5-e109d9cbf62c.png)

The height of the stage is held to that of a taller image. 

The reason that the stage gets the height of the taller images is due it's ```display``` property being set to ```block```. The images are displayed or hidden by toggling the ```z-index``` of the images. My pull request changes the strategy to toggle the images' ```display``` values between ```block``` and ```none``` in order to erase the impact of hidden images on the slide stage.

It appears that this strategy was changed between 2.0 and 2.1. ```z-index``` didn't seem to be integral to the hiding/displaying of images in 2.0 and perhaps is the reason why this regression was introduced.

Additionally, I see the same height issue when using the slide transition; however, that seems to have been a problem since at least 2.0 and does not seem to be a regression in 2.1. This pull request does not address that issue.

Let me know what you think and how I can help get this resolved!

Relevant issues:
woothemes/FlexSlider#426
woothemes/FlexSlider#364
woothemes/FlexSlider#436