Merge concurrent http request to same url into single req. Use native animation.

Hi. Thanks for a great library! I wrote a couple of fixes. Please merge if you find these commits useful.

1) Multiple concurrent requests to same URL were fired if the same image was shown multiple times on the screen.
2) NineOldAndroids has a memory leak. I don't remember the details anymore. It was a few months ago when I traced the problem. Anyway, let's use native animation if it is available. And anyway, maybe it's not even worth including a big library for such a minor fade in animation.