Add ability to request customs form documentation and specify SoldTo parameters

This makes the following changes:

Ability to specify SoldTo in the request in regards to customs documentation
Ability to request invoice forms (FormType 01 only) by specifying product and other related information, and receive the image of that documentation
Improves shipping flow spec to be structured a little better and more readable
Adds label_graphic_image, label_html_image, label_graphic_extension. The non-prefixed versions have remained for backwards compatibility reasons
Improved examples in README file to fit with the latest changes, and add more shipping examples
Added .DS_Store to gitignore file (unrelated)

veeqo / ups-ruby