Fixes to Spree::TaxCloud#destination_address

I ran into a couple issues running spree_tax_cloud on ruby 2.0.0 and spree 2.0.3.

TaxCloud::Address was attempting to refer to Spree::TaxCloud::Address, which caused an error. Replaced with explicit ::TaxCloud::Address

taxcloud_usps_user_id was an empty string after saving the configuration. Updated the test to check that it is .present?

TaxCloud::Address was then initialized using Capitalized strings, which the tax_cloud gem doesn't seem to take
https://github.com/drewtempelmeyer/tax_cloud