;;; It is the opposite of fill-paragraph    
(defun unfill-paragraph ()
  "Takes a multi-line paragraph and makes it into a single line of text."
  (interactive)
  (let ((fill-column (point-max)))
    (fill-paragraph nil)))

And to bind it to a key:

;; Handy key definition
(define-key global-map "\M-Q" 'unfill-paragraph)

Most of the GNU Emacs integrated environment is written in the programming language called Emacs Lisp. The code written in this programming language is the software—the sets of instructions—that tell the computer what to do when you give it commands. Emacs is designed so that you can write new code in Emacs Lisp and easily install it as an extension to the editor.

(GNU Emacs is sometimes called an “extensible editor”, but it does much more than provide editing capabilities. It is better to refer to Emacs as an “extensible computing environment”. However, that phrase is quite a mouthful. It is easier to refer to Emacs simply as an editor. Moreover, everything you do in Emacs—find the Mayan date and phases of the moon, simplify polynomials, debug code, manage files, read letters, write books—all these activities are kinds of editing in the most general sense of the word.)

A preview of the full source code.