Add StandardBurnableToken implementation (#870)

* Add StandardBurnableToken implementation

BurnableToken that extends from StandardToken and adds a
burnFrom method that decrements allowance. Equivalent to
a transferFrom plus burn in a single operation.

* Return event object from expectEvent helper

* Add comment on Approval event in burnFrom function

* Improvements on burnable token tests

- Inject initial balance as a parameter to the behaviour
- Use expectEvent helper for assertions on events
- Use chai bignumber for numbers
- Change to bdd-style assertions

test/helpers/expectEvent.js

@@ -3,6 +3,7 @@ const assert = require('chai').assert;
 const inLogs = async (logs, eventName) => {
   const event = logs.find(e => e.event === eventName);
   assert.exists(event);
+  return event;
 };
 
 const inTransaction = async (tx, eventName) => {