Fix demo to prevent memory leak on device orientation change

[massimobio]

This is as recommended in #41
I verified that the leak is fixed with this change.
I also updated the wiki: https://github.com/daimajia/AndroidImageSlider/wiki/Start-Using

[massimobio]

@zhouqiangWang I was just looking at your #103
Does the code in destroySelf() do the same as this fix?
If yes, I'll close this and undo the changes in the wiki.

[daimajia]

@massimobio There are still some problems exists in #103, I'll merge this pull request firstly.

[massimobio]

@daimajia ok, thank you. I'll keep an eye on #103 and see how it develops. If I understand correctly, the destroySelf() method does the same thing as this PR but doesn't require the user to add the extra code, so it might be a cleaner solution.