Make headers conform to package.el requirements

The source is not currently 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.

Background: I'm adding a [MELPA](http://melpa.milkbox.net/) recipe for `bufshow`. :-)

-Steve