Fixing #6: adding a first class service standards lookup

I hope I'm doing this pull request properly...

I tested a few zipcodes and this code appears to work. Unlike other USPS apis, this API doesn't take a list of routes to work on it just takes one so some of the usual "adding to a stack" stuff doesn't fully make sense. I'm open to feedback on how to improve that.

I'm also open to ideas on naming. "FirstClassServiceStandards" is a really long name and not super descriptive of what is happening, but it does match the name used by USPS.

It was amazingly simple to write this integration - thanks for creating such a great/simple framework :thumbsup: