Collapse Maven and Gradle setup into one section with switchable examples

This commit adds an Asciidoctor extension that post-processes code
blocks, collapsing any secondary blocks into the preceding primary
block. The primary block is then augmented with a switch. This switch
contains one item for each block. Clicking an item causes the associated
block’s content to be displayed. Each item is named using its block’s
title.

The getting started instructions have been updated to take advantage of
this new switching support, with the Maven and Gradle instructions
being collapsed into a single section.

Closes gh-189

buildSrc/src/main/resources/codeBlockSwitch.css

@@ -0,0 +1,23 @@
+.hidden {
+	display: none;
+}
+
+.switch {
+	border-width: 1px 1px 0 1px;
+	border-style: solid;
+	border-color: #7a2518;
+	display: inline-block;
+}
+
+.switch--item {
+	padding: 10px;
+	background-color: #ffffff;
+	color: #7a2518;
+	display: inline-block;
+	cursor: pointer;
+}
+
+.switch--item.selected {
+	background-color: #7a2519;
+	color: #ffffff;
+}