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

contracts/token/ERC20/BurnableToken.sol

@@ -16,14 +16,17 @@ contract BurnableToken is BasicToken {
    * @param _value The amount of token to be burned.
    */
   function burn(uint256 _value) public {
-    require(_value <= balances[msg.sender]);
+    _burn(msg.sender, _value);
+  }
+
+  function _burn(address _who, uint256 _value) internal {
+    require(_value <= balances[_who]);
     // no need to require value <= totalSupply, since that would imply the
     // sender's balance is greater than the totalSupply, which *should* be an assertion failure
 
-    address burner = msg.sender;
-    balances[burner] = balances[burner].sub(_value);
+    balances[_who] = balances[_who].sub(_value);
     totalSupply_ = totalSupply_.sub(_value);
-    emit Burn(burner, _value);
-    emit Transfer(burner, address(0), _value);
+    emit Burn(_who, _value);
+    emit Transfer(_who, address(0), _value);
   }
 }