How to automatically install Emacs packages by specifying a list of package names?

I am using package to manage my Emacs extensions. In order to synchronize my Emacs settings on different computers, I'd like a way to specify a list of package names in .emacs file and then package could automatically search and install the packages, so that I don't need to install them manually by calling M-x package-list-packages. How to do that?

Solution

Based on comments by Profpatsch and answers below:

(defun ensure-package-installed (&rest packages)
  "Assure every package is installed, ask for installation if it’s not.

Return a list of installed packages or nil for every skipped package."
  (mapcar
   (lambda (package)
     ;; (package-installed-p 'evil)
     (if (package-installed-p package)
         nil
       (if (y-or-n-p (format "Package %s is missing. Install it? " package))
           (package-install package)
         package)))
   packages))

;; make sure to have downloaded archive description.
;; Or use package-archive-contents as suggested by Nicolas Dudebout
(or (file-exists-p package-user-dir)
    (package-refresh-contents))

(ensure-package-installed 'iedit 'magit) ;  --> (nil nil) if iedit and magit are already installed

;; activate installed packages
(package-initialize)