* 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
