Allowing configuration through _config.yml

So, as I said in #3, this is a implementation for allowing to configure compass throug the default jekyll `_config.yml` file. It's pretty simple, since it's just a wrapper for the compass configurations.

e.g.

```yml
name: kelvinst blog

gems:
  - jekyll-compass

compass:
  images_path: img
  generated_images_path: img
```

The old fixed/hardcoded configurations were assumed as the default values if none configuration was given.

So, what do you think @mscharley?