Add constructors in JarPluginManager

pf4j/src/main/java/org/pf4j/JarPluginManager.java

  */
 package org.pf4j;
 
+import java.nio.file.Path;
+
 /**
  * It's a {@link PluginManager} that loads each plugin from a {@code jar} file.
  * Actually, a plugin is a fat jar, a jar which contains classes from all the libraries,
  */
 public class JarPluginManager extends DefaultPluginManager {
 
+    public JarPluginManager() {
+        super();
+    }
+
+    public JarPluginManager(Path pluginsRoot) {
+        super(pluginsRoot);
+    }
+
     @Override
     protected PluginDescriptorFinder createPluginDescriptorFinder() {
         return new ManifestPluginDescriptorFinder();