Use absolute import paths for easier markdown integration

Convert relative to absolute import paths to allow using the module from outside markdown’s extension directory.
Using the pull request version you can install AbsoluteImagesExtension like this:

```shell
pip install markdown
pip install -e git+https://github.com/ana-balica/absolute_images#egg=absolute_images
```

And register it with markdown like this:

```python
from markdown import Markdown
from absolute_images import AbsoluteImagesExtension

img_pths = AbsoluteImagesExtension(configs=[['base_url', '/static/images/']])

md = Markdown(extensions=[img_pths,])
md.convert('![Test Image](test.png)')
# u'<p><img alt="Test Image" src="/static/images/test.png" /></p>'
```

As I’m not familiar with Markdown’s extension API, I might have missed a default way how extensions should be installed.