Fix autoloads so the entire package is not loaded unconditionally

The use of `;;;###autoload` with the `defpowerline` macro caused `powerline.el` to be loaded unconditionally at startup. This commit prevents that misbehaviour.

P.S. I noticed this because my Emacs was jumping around slightly when it received focus, and I was unaware of having altered `focus-in-hook`.  When I looked, I found powerline's function in there.  So separately, there may be a problem with the focus function. :-)