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/mocks/StandardBurnableTokenMock.sol

@@ -0,0 +1,13 @@
+pragma solidity ^0.4.18;
+
+import "../token/ERC20/StandardBurnableToken.sol";
+
+
+contract StandardBurnableTokenMock is StandardBurnableToken {
+
+  function StandardBurnableTokenMock(address initialAccount, uint initialBalance) public {
+    balances[initialAccount] = initialBalance;
+    totalSupply_ = initialBalance;
+  }
+
+}