Handling of CSS resource paths

Hi - thanks for a great tool! Planning to use it for a PyCon talk.

I've noticed an issue with the calculation of paths for resources referenced with ``url()`` in CSS, when the CSS file is placed in a subdirectory rather than adjacent to the presentation source. An example is the easiest way to demonstrate:

```
slides.rst
css/
    styles.css
fonts/
    somefont.eot
```

For this structure to work in the final presentation, the CSS must use a relative reference like ``url('../font/somefont.eot')``. But hovercraft's resource-copying code interprets these relative paths as relative to the slides source, not relative to the location of the CSS file, so hovercraft is unable to find that font file to copy when generating the presentation.

I can submit a pull request to fix this, just wanted to verify first that this is considered a bug and a fix would be accepted.