SPDisplayObject's self.stage should be deprecated

This property is a major cause for confusion and [SPStage mainStage] should always
be used instead. I have seen at least 9 threads about this in the Sparrow Forum, because people are confused about which method should be used to access the stage. I have also updated the documentation to remove references to self.stage.

http://forum.sparrow-framework.org/topic/animation-in-playc-file#post-11012
http://forum.sparrow-framework.org/topic/name-property-issue#post-8221
http://forum.sparrow-framework.org/topic/issues-with-tweenning-and-the-juggler#post-10533
http://forum.sparrow-framework.org/topic/spmovieclip-not-animated-even-after-a-self#post-6995
http://forum.sparrow-framework.org/topic/no-spjuggler#post-9705
http://forum.sparrow-framework.org/topic/movieclip-doesnt-play#post-10475
http://forum.sparrow-framework.org/topic/problem-with-tweens-in-substage-tweens-do-not-start#post-7962
http://forum.sparrow-framework.org/topic/spmovieclip-not-playing#post-11507
http://forum.sparrow-framework.org/topic/sptween-nao-funciona-na-classe-spsprite#post-10226

Thanks for your time,
Brian