Add helpful constructor from a multihash

Author: ianopolous

src/main/java/io/ipfs/multiaddr/MultiAddress.java

@@ -1,12 +1,18 @@
 package io.ipfs.multiaddr;
 
+import io.ipfs.multihash.*;
+
 import java.io.*;
 import java.util.*;
 
 public class MultiAddress
 {
     private final byte[] raw;
 
+    public MultiAddress(Multihash hash) {
+        this("/ipfs/" + hash.toBase58());
+    }
+
     public MultiAddress(String address) {
         this(decodeFromString(address));
     }