Added prefixed css-rules when resetting transform on showIFrame

[kaarholt]

CSS vendor prefixes were added in hideIFrame (via ReadiumSDK.Helpers.CSSTransformString), but not removed in showIFrame.
This resulted in the iframe remaining 'invisible' even after showIframe.

[danielweck]

Thank you Kristoffer, much appreciated.
I totally agree with you, and there is in fact an issue about harmonising how CSS prefix are handled, so that this kind of CSS property "toggle" bug does not occur. See: #135

[danielweck]

@kaarholt PS: on what platform did you observe the "invisible iframe" bug?

[kaarholt]

This was on mac, with latest Chrome canary build.
I didn’t test in more browsers since the reason for the bug seemed obvious.

The value for '-webkit-transform: ….' remained on the iframe while only the unprefixed ‘transform’ got reset to ‘none’.
Since the prefixed rule came AFTER the unprefixed rule in the style string, it became the actual rule that Chrome used.

[danielweck]

Hello @kaarholt
I have merged a "complete" fix into develop: #137
Could you please confirm that it works on your end?
Many thanks!
PS: I tested on OSX, Safari, Chrome and WebView