api: wrap transport and add resources to backend

What is currently available as `vulcano` inside resources (e.g. to call `vulcano.file(...)`, is now wrapped inside `vulcano.backend`. All other resources are now added to `vulcano.<RESOURCE>`, e.g. `vulcano.user`.

lib/resources/command.rb

@@ -9,7 +9,7 @@ class Cmd < Vulcano.resource(1)
   end
 
   def result
-    @result ||= vulcano.run_command(@command)
+    @result ||= vulcano.backend.run_command(@command)
   end
 
   def stdout

lib/resources/file.rb

@@ -8,7 +8,7 @@ module Vulcano::Resources
 
     def initialize(path)
       @path = path
-      @file = vulcano.file(@path)
+      @file = vulcano.backend.file(@path)
     end
 
     %w{

lib/resources/os.rb

@@ -5,7 +5,7 @@ module Vulcano::Resources
     name 'os'
 
     def [](name)
-      vulcano.os[name]
+      vulcano.backend.os[name]
     end
   end
 end

lib/vulcano/runner.rb

@@ -36,8 +36,11 @@ module Vulcano
       RSpec.configuration.add_formatter(@conf['format'] || 'progress')
     end
 
-    def add_resources_to_backend(backend_class)
-      Class.new(backend_class) do
+    def create_backend_container(backend_instance)
+      Class.new do
+        define_method :backend do
+          backend_instance
+        end
         Vulcano::Resource.registry.each do |id, r|
           define_method id.to_sym do |*args|
             r.new(self, *args)
@@ -60,8 +63,9 @@ module Vulcano
       end
 
       # create the backend based on the config
-      enriched_backend = add_resources_to_backend(backend_class)
-      @backend = enriched_backend.new(@conf)
+      backend_instance = backend_class.new(@conf)
+      outer_cls = create_backend_container(backend_instance)
+      @backend = outer_cls.new
     end
 
     def add_tests(tests)