Move shell-pop from bottom to top.

Having it at the bottom is more faithful to GEdit, but unfortunately I
am experiencing really bad interactions with sr-speedbar here. What's
happening is that every file I open with sr-speedbar gets opened in
the teeny shell-pop bottom window rather than the much larger top
window, where I want them to open. I've been googling this issue for
days, and I just cannot find any info at all about changing this
behavior, so I simply give up on fighting it. By making shell-pop open
at the top, sr-speedbar still opens files in the bottom window, which
is now the window that I want files opened in.