Add a BuildInfo task for generating build.properties with Gradle

The commit adds a new BuildInfo task that can be used to generate
a build.properties file, intended for inclusion in the Actuator's
info endpoint.

A default instance of the task can be configure using the plugin's
DSL:

springBoot {
	buildInfo()
}

Additional properties can also be configured using the DSL:

springBoot {
	buildInfo {
		additionalProperties = [
			'foo': 'bar'
		]
	}
}

When configured via the DSL, the Java plugin's classes task is
configured to depend on the build info task. Alternatively, if more
control is required, the task can be declared and configured manually:

task buildInfo(type: org.springframework.boot.gradle.buildinfo.BuildInfo) {
	additionalProperties = [
		'foo': 'bar'
	]
}

classes {
	dependsOn buildInfo
}

See gh-2559

spring-boot-samples/spring-boot-sample-actuator/build.gradle

@@ -22,9 +22,11 @@ apply plugin: 'spring-boot'
 
 jar {
 	baseName = 'spring-boot-sample-actuator'
-	version =  '0.0.0'
 }
 
+group = 'org.springframework.boot'
+version = springBootVersion
+
 repositories {
 	// NOTE: You should declare only repositories that you need here
 	mavenLocal()
@@ -59,3 +61,7 @@ springBoot {
 task wrapper(type: Wrapper) {
 	gradleVersion = '1.6'
 }
+
+springBoot {
+	buildInfo()
+}