* 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