Fix whitespace
Rename #compute methods to #compute_package

* The superclass adds a #compute method that coerces the object into
  a package.
Add Spree::Calculator::Shipping::ActiveShipping::Base#available?

* This method will return true if the shipping method is available.
  There are cases where the API does not always return a rate for a
  given service, and this is needed so that it will be skipped over.

Fixes #131