JavascriptHeader module

I've created a helper module named `JavascriptHeader`, which behaves like `LinkModule`, adding the `javascript` method, which returns script tags for linking javascript files.

Usage (ie in a slim template):

```ruby
doctype html
head
  == javascript '/path/to/file.js'
body
  h1 Hello World
```

The result will be something like:

```html
<!DOCTYPE html>
<html>
  <head>
    <script src="/path/to/file.js" type="text/javascript"></script>
  </head>
  <body>
    <h1>Hello World</h1>
  </body>
</html>
```