Custamizable isearch keybinding

I replaced a not good use of hook to register isearch keybinding.
That was the root of less customizability because
  1. nameless function can't be remove by `remove-hook';
  2. body, defining keybiding, called each time isearch starts.
     For example,
       (define-key isearch-mode-map "\C-y" nil)
     in .emacs file has no effect. (Overrided by the hook.)
Improve infomation of a customizable variable
Merge pull request #10 from kenoss/master

Custamizable isearch keybinding