Fix #9 Background color

[abousquet]

I have added the :background_color option to both SVG and PNG images. Both image types default this option to white. SVG's implementation allows for transparency as well by passing the :transparent as the value.

Work left to do:

• Add transparency to PNG images as well
• Create visual/manual verification tests for both image types

[abousquet]

I've added two test modules for PNG and SVG, respectively. They generate an html page in test/html that renders generated QR codes that point to www.google.com with various options. The PNG files had to be saved to disk in test/images. Both of these directories are created if they do not exist and are ignored by git.

I've added transparency to PNG by changing the color type from 2 to 6, in order to add an extra byte for alpha channel.

Lastly, I cleaned up the SVG module to use elixir pattern matching and the pipe operator.

[nthock]

Thanks for the PR. Will update the hex package soon with this commit.