Add Package-Requires header for ELPA installations

Hi, I plan to add this to [MELPA](http://melpa.milkbox.net/).

When installed as a package from an ELPA archive such as [MELPA](http://melpa.milkbox.net/), this header tells `package.el` to also install `dash`, which the library requires. For information about this fix, See [Library-Headers](http://www.gnu.org/software/emacs/manual/html_node/elisp/Library-Headers.html).