ob-core: Update allowed values for :eval header arg

* lisp/ob-core.el (org-babel-common-header-args-w-values): Add allowed
  values for :eval arg.

Reported-by: Thomas S. Dye <tsd@tsdye.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/97272>

lisp/ob-core.el

@@ -461,7 +461,7 @@ then run `org-babel-switch-to-session'."
     (colnames	. ((nil no yes)))
     (comments	. ((no link yes org both noweb)))
     (dir	. :any)
-    (eval	. ((never query)))
+    (eval	. ((yes no-export strip-export eval never query)))
     (exports	. ((code results both none)))
     (epilogue   . :any)
     (file	. :any)