Generate fallback CSS automatically

[osuushi]

To support older browsers (which don't allow data URIs to be used this way), and browsers with JS disabled, sixpack should be able to generate a CSS file that points to the original images files.

Requirements:

1. Allow user to specify the base URL for the image directory in sixpack
2. Generate a style sheet which points to those images, exactly replicating the end result of using the sixpack JSON file
3. Generate a block of code to insert in the HTML file which will enable the fallback.

• noscript code embedding the style sheet
• call to LemonSoda method to trigger feature detection

4. Feature detection for using data URIs in LemonSoda