Return IPFS hash in addContribution() response

Needed in kredits-web. Useful for clients to identify contributions
before knowing their assigned ID.

lib/contracts/contribution.js

@@ -58,7 +58,10 @@ class Contribution extends Record {
           ipfsHashAttr.hashSize,
         ];
 
-        return this.functions.add(...contribution, callOptions);
+        return this.functions.add(...contribution, callOptions).then(res => {
+          res.ipfsHash = this.ipfs.encodeHash(ipfsHashAttr);
+          return res;
+        });
       });
   }