Added USPS First Class Package International Service
Merge pull request #156 from dpritchett/master

Added USPS First Class Package International Service Calculator
max weight for country only returns oz - refactored max_weight_for_country - changed some services available method to max_weight_for_country, this will avoid getting zero results back to the estimator because available? in super now checks the api response - removed hard coded * 16 from hell

Conflicts:
	app/models/spree/calculator/shipping/usps/first_class_mail_international.rb
	app/models/spree/calculator/shipping/usps/first_class_mail_international_large_envelope.rb
	app/models/spree/calculator/shipping/usps/first_class_mail_parcels.rb

Conflicts:
	app/models/spree/calculator/shipping/usps/first_class_mail_international.rb
	app/models/spree/calculator/shipping/usps/first_class_mail_international_large_envelope.rb
Clarify weights and carrier rates - Added comments on what units are expected back from Spree - Example on converting units from kb to oz - Clarify how the carrier rates are obtained and cached
first check if a package is shippable this happens before calling the carrier - Make sure the available? overrides inside each service don't raise exceptions
changed weight limits from lb to oz on usps calcs this extension works with oz and this services where long using lbs erroneously
fixed weight spec package that had to have weight and had to have an international address
changed ContentItem parameter requirements this changed in spree_core in #3991
Add the product package javascripts to the precompile list.

Signed-off-by: Nathan Lowrie <nate@finelineautomation.com>

Fixes #162