Fixed autoloading issue.

Hey, I noticed that when I pulled this in using composer that it wasn't able to properly resolve the classes with the code as-is unless (for example) I moved the source code to a "USPS" sub directory. I figured instead it would be best to update the composer.json reference in the package itself to use psr-4 and setup a namespace prefix with the root path so it'd load for me.

Did you have any issues with this before?