Adding the possibility to exclude invisible lines and patches in relim

Following the suggestion from John
http://sourceforge.net/mailarchive/forum.php?thread_name=4F5FB1F7.9050909%40hawaii.edu&forum_name=matplotlib-users

I included the visible_only argument, to be able to perform a relim with only the visible elements.

I didn't update the docstring because I do not know I should replace the whole docstring for the new format or how to mix and match new/old format.