All user variables now converted to custom.

All user variables now converted to custom.
(iswitchb): New variable for use by the custom package.
(iswitchb-default-buffer): Variable deleted.
(iswitchb-define-mode-map): Addition of keybindings
for iswitchb-kill-buffer and iswitchb-find-file.
(iswitchb): When no text typed in, show all buffers.
(iswitchb-complete): Use equal rather than eq.
(iswitchb-next-match, iswitchb-prev-match): Use
iswitchb-chop to handle reordering the buffer list.
(iswitchb-chop): New function.
(iswitchb-make-buflist): Rewritten for efficiency.
(iswitchb-to-end): Operate on a list of buffers, not just one.
(iswitchb-set-matches): Always return list
of matching buffers, even in absence of user input.
(iswitchb-kill-buffer): New function.
(iswitchb-default-keybindings): Use read-kbd-macro for keys to define.
(iswitchb-exhibit): Always return list of matching buffers.
(iswitchb-show-default-buffer): Function deleted.