Add package `elisp-slime-nav'

Remove function `aethanyc-find-at-point' which provides similar
functionality as `elisp-slime-nav'.
Specify modes that need not to be saved
Reconsider `switch-window' keybindings

* `switch-window' supersedes `other-window', so I remap `other-window'
   keys to `switch-window'.
* I do not like ibuffer's "M-o" key, so I rebind "M-o" to
  `switch-window' in all modes.