Make headers conform to package.el requirements

The source is not parseable by `package-buffer-info`, which
renders it uninstallable from a single-file ELPA package. This
commit fixes that, and adds an explicit dependency on Emacs
>= 24, since `lexical-binding` is not available in older versions.
Merge pull request #1 from purcell/patch-1

Make headers conform to package.el requirements