Explicitly require emacs 24, and tidy up package description

This commit adds an explicit package dependency upon Emacs 24
(see js2-mode for another example of this), and moves the
lexical-bindings variable clause out of the package description,
which prevents it from showing up in `package-list-packages`.
Merge pull request #2 from purcell/patch-1

Explicitly require emacs 24, and tidy up package description