Refactor kredits address initialization

This moves the Kredits initialization to the instance which allows us to
be more flexible with handling contract addresses.

Example:

var k = new Kredits(provider, signer, {Registry: '0xabc'});
k.init().then((kredits) { ...});

var k = new Kredits(provider, signer, {Contributors: '0xabc'})
k.Contributor.add(...);

lib/contracts/index.js

@@ -1,5 +1,6 @@
 module.exports = {
   Contributors: require('./contributor'),
   Operator: require('./operator'),
-  Token: require('./token')
-}
+  Token: require('./token'),
+  Registry: require('./registry')
+};

lib/contracts/registry.js

@@ -0,0 +1,6 @@
+const Base = require('./base');
+
+class Registry extends Base {
+}
+
+module.exports = Registry;