"GONE" viewBehind to prevent taking part in onMeasure() when SlidingMenu closed

Hi!

Just noticed when you "INVISIBLE" view behind it wastes some time for onMeasure even if it's hidden.
So, I tried to "GONE" view and it become much better for performance in ours particular application.

Please, checkout this pull request and leave you comments, may be I'm wrong and you know smth. that I don't :).

Sorry, don't know how to make pull request for commit, so check following commit: "5fdeab1db951d4c520657cdfd702ee7ecabaf3ef", all changes in there.

Thanks!