rate calculations per package not by order
added some helper functions as well to cleanup some of the code
also made sure the weight tests where using packages instead
of orders and that they still made sense

Conflicts:
	spec/models/weight_limits_spec.rb
Merge branch '2-1-stable' into calculate_rates_per_package_2_1_stable

* 2-1-stable:
  fix spree version dependency
Merge pull request #142 from mrpollo/calculate_rates_per_package_2_1_stable

Calculate rates per package