Tidy up package description by introducing local variables block

Hi! I've just added a recipe for `lexbind-mode` to [MELPA](http://melpa.milkbox.net).

However, setting variables in the first line messes up the package lists produced by Marmalade and MELPA -- see below.

```
  auto-highlight-... 1.55         new        Automatic highlighting current symbol minor mode
  dot-mode           1.12         new        minor mode to repeat typing or commands
  lexbind-mode       0.5          new        Puts the value of lexical-binding in the mode line ;; -*- lexical-binding: t; -*-
  abl-mode           20120718.... available  Python TDD minor mode [source: github]
```

This commit moves the `lexical-binding` declaration to a local variables block.