babel: adding :noeval header argument to inhibit execution of code blocks on export

* contrib/babel/lisp/org-babel.el (org-babel-header-arg-names): adding
  :noeval header argument which can be specified to inhibit the
  execution of a source block during export.

* contrib/babel/lisp/org-babel-exp.el (org-babel-exp-do-export):
  adding :noeval header argument which can be specified to inhibit the
  execution of a source block during export.

contrib/babel/lisp/org-babel-exp.el

@@ -122,7 +122,9 @@ options are taken from `org-babel-default-header-args'."
   "Return a string containing the exported content of the current
 code block respecting the value of the :exports header argument."
   (flet ((silently () (let ((session (cdr (assoc :session (third info)))))
-			(when (and session (not (equal "none" session)))
+			(when (and session
+				   (not (equal "none" session))
+				   (not (assoc :noeval (third info))))
 			  (org-babel-exp-results info type 'silent))))
 	 (clean () (org-babel-remove-result info)))
     (case (intern (or (cdr (assoc :exports (third info))) "code"))

contrib/babel/lisp/org-babel.el

@@ -93,7 +93,8 @@ then run `org-babel-pop-to-session'."
 (add-hook 'org-metadown-hook 'org-babel-pop-to-session-maybe)
 
 (defconst org-babel-header-arg-names
-  '(cache cmdline colnames dir exports file noweb results session tangle var)
+  '(cache cmdline colnames dir exports file noweb results
+	  session tangle var noeval)
   "Common header arguments used by org-babel.  Note that
 individual languages may define their own language specific
 header arguments as well.")