Handle bignumber conversions

The kredits module always returns raw data from the contract. For uint
values these are bignumbers.
To handle those in the ember app we need to convert them to string or to
number.

app/models/contributor.js

@@ -1,12 +1,13 @@
 import computed from 'ember-computed';
 import EmberObject from 'ember-object';
+import bignumber from 'kredits-web/utils/cps/bignumber';
 
 export default EmberObject.extend({
   // Contract
-  id: null,
+  id: bignumber('idRaw', 'toString'),
   // TODO: Should we rename it to account like in the contract?
   address: null,
-  balance: 0,
+  balance: bignumber('balanceRaw', 'toNumber'),
   isCore: false,
   ipfsHash: null,