Assets can be disabled

In a plugin's configuration you can disable any plugin asset like this:

```yml
disable:
  images: true      # disables all assets of this type
  pages: test.html  # disable a single asset
  files:            # disable a specific set of files
   - robots.txt
   - sitemap.xml
```

CLI Updates

To make this work well, the output of `octopress ink info` has been
improved.

Running `octopress ink info --all` will display each plugin followed by
the assets for that plugin.

Running `octopress ink info --javascripts --images` will show each plugin's
javascripts and images. This should be helpful if you have lots of
plugins and are trying to figure out where a specific asset is coming
from.

If an asset is disabled it will show up like this:

```
 Sass:
  - main.scss                          disabled
```

Additionally since pages can be configured to have different output
paths, I now display the output path with the asset info.

```
 Pages:
  - test_pages/one.xml                 output: test_pages/feed/index.xml
  - test_pages/three.md                output: test_pages/three.html
  - test_pages/two.md                  output: test_pages/theme_page.html
```

I still have some tests to write for this, but it's coming along.
Removed default_url from page class as it was not being used
Removed unused disable method from asset class
Added tests for disabled assets.