Initial legwork for Spree 2.0 compatibility

To @rterbush with warm regards!

# Why you should accept this :thumbsup: 
* It renders a simple invoice PDF under 2.0 unlike before
* Updated asset path and l10n file structures to match 2.0.
* It'll save you an hour or so of work getting started down the same path

# Why you should not accept this :thumbsdown: 
* Intentionally blows up when more than one stock location is in play - not sure how or where to hook in for multi-shipment use cases
* The test suite seems to need to be adjusted to run under Spree 2.0
* Only tested for a single case under Spree 2.0.0 in English
* `Versionfile` has not been properly updated

Thanks!