getTransactionHash method is now static

- This method has no state or reliance on the builder itself, so it
  is made static for convenience.

src/main/SignedTransactionBuilder.java

@@ -217,7 +217,7 @@ public byte[] buildSignedTransaction() throws InvalidKeySpecException, InvalidKe
      * @throws NullPointerException if signedTransaction is null.
      * @throws IllegalStateException if the provided bytes could not be interpreted.
      */
-    public byte[] getTransactionHashOfSignedTransaction(byte[] signedTransaction) {
+    public static byte[] getTransactionHashOfSignedTransaction(byte[] signedTransaction) {
         if (signedTransaction == null) {
             throw new NullPointerException("cannot extract hash from a null transaction.");
         }