Add contract address accessor

lib/contracts/base.js

@@ -7,6 +7,10 @@ class Base {
     return this.contract.functions;
   }
 
+  get address () {
+    return this.contract.address;
+  }
+
   get ipfs () {
     if (!this._ipfsAPI) { throw new Error('IPFS API not configured; please set an ipfs instance'); }
     return this._ipfsAPI;