Modularize asciidoc documentation

The reference documentation is now organized in asciidoc bookparts.
Bookparts sections can be also put in separate files, when their
size and theme are important enough to justify that.

Documentation Layout:
```
index
  |-- overview
  |
  |-- whats-new
  |
  |-- core
  |   |-- core-beans
  |   |-- core-aop
  |-- testing
  |
  |-- data-access
  |
  |-- web
  |   |-- web-mvc
  |   |-- web-view
  |-- integration
  |
  |-- appendix
      |-- appx-spring-tld
      |-- appx-spring-form-tld
```

Supersedes and closes #641

Issue: SPR-12309

src/asciidoc/Guardfile

@@ -7,7 +7,10 @@ FileUtils.cp_r('images','build')
 
 guard 'shell' do
   watch(/^.*\.adoc$/) {|m|
-    Asciidoctor.render_file(m[0], :to_dir => "build", :safe => Asciidoctor::SafeMode::UNSAFE, :attributes=> {'idprefix' => '', 'idseparator' => '-', 'copycss' => '', 'icons' => 'font', 'source-highlighter' => 'prettify', 'sectanchors' => '', 'doctype' => 'book','toc2' => '', 'spring-version' => '4.0.0.BUILD-SNAPSHOT', 'revnumber' => '4.0.0.BUILD-SNAPSHOT' })
+  	if m[0] != "index.adoc"
+  	Asciidoctor.render_file(m[0], :to_dir => "build", :safe => Asciidoctor::SafeMode::UNSAFE, :attributes=> {'idprefix' => '', 'idseparator' => '-', 'copycss' => '', 'icons' => 'font', 'source-highlighter' => 'prettify', 'sectanchors' => '', 'doctype' => 'book', 'toc' => 'left', 'toclevels' => '2', 'spring-version' => '4.2.0.BUILD-SNAPSHOT', 'revnumber' => '4.2.0.BUILD-SNAPSHOT' })
+  	end
+    Asciidoctor.render_file("index.adoc", :to_dir => "build", :safe => Asciidoctor::SafeMode::UNSAFE, :attributes=> {'idprefix' => '', 'idseparator' => '-', 'copycss' => '', 'icons' => 'font', 'source-highlighter' => 'prettify', 'sectanchors' => '', 'doctype' => 'book', 'toc' => 'left', 'toclevels' => '1', 'spring-version' => '4.2.0.BUILD-SNAPSHOT', 'revnumber' => '4.2.0.BUILD-SNAPSHOT' })
   }
 end
 

src/asciidoc/index-docinfo.xml

 <productname>Spring Framework</productname>
 <releaseinfo>{revnumber}</releaseinfo>
 <copyright>
-	<year>2004-2014</year>
+	<year>2004-2015</year>
 </copyright>
 <legalnotice>
 	<para>Copies of this document may be made for your own use and for distribution to