Refactoring digital shipping calculation and display

* removed the deface override that modified the checkout/_delivery partial
* created custom digital delivery calculator
* removed digital_shipping_method from Spree::Order. Moved logic to available_shipping_methods monkey patch

The main goal was to eliminate the shipping method detection logic from the _delivery partial.
This was a bit of a hack all along and didn't really present the correct information to the underlying
order logic (i.e. rate_hash.blank? would return true when there was no shipping options available).
This should be more future proof and make overriding another aspect of the delivery process easier.

spree_digital.gemspec

@@ -1,9 +1,9 @@
 Gem::Specification.new do |s|
   s.platform     = Gem::Platform::RUBY
   s.name         = 'spree_digital'
-  s.version      = '1.0.0'
+  s.version      = '1.1.0'
   s.summary      = ''
-  s.description  = 'This gem is supposed to be used in connection with spree_core. It was tested with spree_core 0.66.99 but it might work with newer versions as well.'
+  s.description  = 'Add digital download functionality to spree'
   s.author       = 'funkensturm.'
   s.homepage     = 'http://www.funkensturm.com'
   s.files        = `git ls-files`.split("\n")
@@ -11,7 +11,7 @@ Gem::Specification.new do |s|
   s.require_path = 'lib'
   s.requirements << 'none'
   s.required_ruby_version = '>= 1.8.7'
-  s.add_dependency('spree_core', '>= 1.0.0')
+  s.add_dependency('spree_core', '>= 1.1.0')
 
   # test suite
   s.add_development_dependency 'shoulda-matchers'