New plugin: blog_images

Hi Koen,

I'm brand new to python, so please excuse any mistakes.

This is a new plugin (inspired by a similar one for docpad called "associated-images") that allows users of the blog.py plugin to place image files in a specially named directory within the static directory. For example:

Given the following file structure:
```
├── static
│   ├── blog-post-images
│   │   └── first-post
│   │       ├── 4torresBackyard.jpg
│   │       ├── BosqueTalladores_Panorama1.jpg
│   │       ├── ChillanSki_Panorama1.jpg
│   │       └── cachiPanorama.jpg
...
├── pages
│   ├── posts
│   │   ├── first-post.html
```

The plugin will allow you to load an array of image paths like so in any page:
```
{% for image in images %}
  <img src="{{ STATIC_URL }}/{{ image.path }}" alt="" />
{% endfor %}
```