Allow 'pry-rails/console' require (fix #27)

You can now use `gem 'pry-rails', require: 'pry-rails/console'` to only
load Pry in the console.

lib/pry-rails/console.rb

@@ -0,0 +1,7 @@
+# encoding: UTF-8
+
+require 'pry-rails/version'
+
+if defined?(Rails)
+  require 'pry-rails/railtie'
+end

lib/pry-rails/railtie.rb

@@ -3,6 +3,9 @@
 module PryRails
   class Railtie < Rails::Railtie
     console do
+      require 'pry'
+      require 'pry-rails/commands'
+
       if Rails::VERSION::MAJOR == 3
         Rails::Console::IRB = Pry
 
@@ -24,4 +27,3 @@ module PryRails
     end
   end
 end
-