Ruta: MultiTreeWordList Constructor

I've had problems using the MultiTreeWordList. I think one constructor misses initialization of the root node.

Possible patch:

Index: MultiTreeWordList.java
===================================================================
— MultiTreeWordList.java (revision 1521373)
+++ MultiTreeWordList.java (working copy)
@@ -132,6 +132,7 @@

• path of the file to create a TextWordList from
  */
  public MultiTreeWordList(String[] pathnames) throws IOException {
  + this.root = new MultiTextNode();
  for (String pathname : pathnames) { load(new FileSystemResource(pathname)); }