Prevent informational JMH messages for default Gradle log level (#265)

Currently every JMH bytecode generation task run produces log messages like the following even if just the default "lifecycle" log level is being used.

```
> Task :jmhRunBytecodeGenerator
Processing 0 classes from .../build/classes/java/jmh with "reflection" generator
Writing out Java source to .../build/jmh-generated-sources and resources to .../build/jmh-generated-resources
Processing 0 classes from .../build/classes/java/test with "reflection" generator
Writing out Java source to .../build/jmh-generated-sources and resources to .../build/jmh-generated-resources
```

This change prevents these log messages unless Gradle's "info" (or finer grained) log level is being used.

src/main/java/me/champeau/jmh/JmhBytecodeGeneratorTask.java

@@ -26,7 +26,9 @@
 import org.gradle.process.ExecOperations;
 
 import javax.inject.Inject;
+import java.io.ByteArrayOutputStream;
 import java.io.File;
+import java.io.OutputStream;
 
 @CacheableTask
 public abstract class JmhBytecodeGeneratorTask extends DefaultTask implements WithJavaToolchain {
@@ -73,6 +75,14 @@ public void generate() {
                         getGeneratedResourcesDir().get().getAsFile(),
                         getGeneratorType().get()
                 );
+
+                // "Disable" stdout if Gradle's log level is not "into" or more verbose to
+                // prevent the just informational messaged from JMH's bytecode generator, like
+                // "Processing 30 classes from ..." and "Writing out Java source to ...".
+                if (!getLogger().isInfoEnabled()) {
+                    spec.setStandardOutput(new ByteArrayOutputStream());
+                }
+
                 spec.jvmArgs(getJvmArgs().get());
                 Provider<JavaLauncher> javaLauncher = getJavaLauncher();
                 if (javaLauncher.isPresent()) {