Generate full paths for images in guides

Suppose we're using JSDuck to generate guides from the following directory
structure:

somedir/
  guides.json
  guides/
    getting_started/
      README.md
      some-image.png
    nested/
      first_guide/
        README.md
        one.png
      second_guide/
        README.md
        one.png

In JSDuck's generated HTML, image tags in the Getting Started guide will point
to the correct path relative to the page (e.g.
'guides/guide_getting_started/some-image.png'). However, the nested guides will
specify the wrong paths to images: JSDuck will write
'guides/first_guide/one.png' instead of the correct
'guides/guide_nested_first_guide/one.png'.

This patch fixes the issue by generating the image's path based on the full
name of the guide, instead of using 'File.basename' to incorrectly crop the
path.