Adding possibility to remove invisible lines and patches from relim

As discussed long time ago in the mailing list, and following the suggestion (copy/paste) from John Hunter http://sourceforge.net/mailarchive/forum.php?thread_name=4F5FB1F7.9050909%40hawaii.edu&forum_name=matplotlib-users

Added the possibility to exclude the invisible lines from relim.
The login behind this is, that if we make a line invisible it is possible that we want to concentrate our attention to the visible ones, and this may require a relim.

I have created a pull-request before, but my inexperience with git, made that I completely screw the branch, so I have to redoit.
Sorry... my bad