Don't rebind C-x C-a in emacs-lisp-mode: it upsets edebug

Fixes #173
Check OS before using exec-path-from-shell for $LEDGER_FILE

See #174
Update README to clarify steps to follow before filing issues
Take my own advice and abbreviate file names in ido-choose-from-recentf
Require 'ido in init-ido
Return to using switch-window on C-x o
Hide lighter for magit-auto-revert-mode