sprues: rename "template" to "sprue"

Also:
  * Move stuff around to a builtin/ folder
  * Separate out classes to follow my one-class-per-file convention
  * Samples are renamed to the underscore convention

I just like the word "sprue", and it's more descriptive than our original choice of "template". If I had to start Squib all over again, I would probably name it "Sprue" :P

bin/squib

@@ -16,12 +16,12 @@ Mercenary.program(:squib) do |p|
     end
   end
 
-  p.command(:make_template) do |c|
-    c.syntax 'make_template'
-    c.description 'Creates a template definition file to generate a templated PDF.'
+  p.command(:make_sprue) do |c|
+    c.syntax 'make_sprue'
+    c.description 'Creates a sprue definition file.'
 
     c.action do |args, options|
-      Squib::Commands::MakeTemplate.new.process(args)
+      Squib::Commands::MakeSprue.new.process(args)
     end
   end