Frame popup support and some small fixes

Frame popup support and some small fixes

Summary of changes:
- add emacs 24.3 as a dependency, because
  `display-buffer-in-major-side-window` doesn't exist in older emacsen
- add option to show which-key buffer in a popup frame
  `(setq which-key-popup-type 'frame)`
- stop close timer when disabling which-key-mode
- don't hide which-key buffer before showing it. instead, show methods
  handle the case that the buffer is already shown
- `which-key/hide-popup` calls function depending on popup
  type (similar to `which-key/show-popup`)
- use `display-buffer-in-major-side-window` instead of
  `display-buffer-in-side-window`, for popup type side-window. ensures
  new side window is created properly even if other side windows already
  exist.
- erase previous contents of which-key buffer before inserting new
  contents, and adjust buffer display in window by moving point to
  the buffer's beginning