Use extracted prawn-manual_builder code

The functionality of the manual should stay the same,
we now just depend on an external gem rather than bundling
the manual building code directly with Prawn.

In theory, this will allow for the manual building code
to be used by other extensions. In practice, some
extra effort may be needed before that's practical.

Rakefile

@@ -32,7 +32,7 @@ desc "Generate the 'Prawn by Example' manual"
 task :manual do
   puts "Building manual..."
   require File.expand_path(File.join(File.dirname(__FILE__),
-    %w[manual manual manual]))
+    %w[manual manifest]))
   puts "The Prawn manual is available at manual.pdf. Happy Prawning!"
 end
 

manual/basic_concepts/basic_concepts.rb

@@ -2,10 +2,10 @@
 #
 # Examples for Prawn basic concepts.
 #
-require File.expand_path(File.join(File.dirname(__FILE__),
-                                   %w[.. example_helper]))
 
-Prawn::Example.generate("basic_concepts.pdf", :page_size => "FOLIO") do
+require_relative "../example_helper"
+
+Prawn::ManualBuilder::Example.generate("basic_concepts.pdf", :page_size => "FOLIO") do
 
   package "basic_concepts" do |p|
 

manual/manual/cover.rb → manual/cover.rb

@@ -2,11 +2,11 @@
 #
 # Prawn manual how to read this manual page.
 #
-require File.expand_path(File.join(File.dirname(__FILE__),
-                                   %w[.. example_helper]))
+
+require_relative "example_helper"
 
 filename = File.basename(__FILE__).gsub('.rb', '.pdf')
-Prawn::Example.generate(filename) do
+Prawn::ManualBuilder::Example.generate(filename) do
   move_down 200
 
   image "#{Prawn::DATADIR}/images/prawn.png",