Provide a default output directory for snippets based on build tool

Rather than requiring an output directory to be explcitly configured,
a default is now automatically configured based on the build tool
that's being used. When using Gradle, snippets will be generated in
build/generated-snippets. When using Maven, snippets will be
generated in target/generated-snippets.

See gh-297

docs/src/test/java/com/example/mockmvc/CustomFormat.java

@@ -30,7 +30,7 @@ import static org.springframework.restdocs.mockmvc.MockMvcRestDocumentation.docu
 public class CustomFormat {
 
 	@Rule
-	public final JUnitRestDocumentation restDocumentation = new JUnitRestDocumentation("build");
+	public final JUnitRestDocumentation restDocumentation = new JUnitRestDocumentation();
 
 	@Autowired
 	private WebApplicationContext context;