ace-window.el: add custom buffer ignoring

* ace-window.el (aw-ignored-buffers): New custom variable.  Set this to
  a list of buffer names that are often around, but you don't want to
  switch to them.

  (aw-ignore-on): New custom variable. This value can be toggled with
  M-0 `ace-window'.

  (aw-ignored-p): New function.
  (aw-list-visual-area): Don't return ignored windows.
  (aw-generic): Add ignoring.
  (ace-window): With 0 prefix argument, toggle `aw-ignore-on'.
ace-window.el (aw-generic): handle zero-length read-only buffers better

Possible fix for #10.