Explicitly call with Spree namespace.
Fixes to Spree::TaxCloud#destination_address

* Refer to ::TaxCloud::Address rather than TaxCloud::Address
* taxcloud_usps_user_id.present? because it can be ""
* TaxCloud::Address is initialized with lowercase symbols
Only calculate eligible adjustments.
Merge pull request #21 from rterbush/eligible-adjustments

Eligible adjustments