token-erc20: add event Approval to follow eip20

> MUST trigger on any successful call to approve(address _spender, uint256 _value).

https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md#approval

We audited many tokens without event Approval.
Should comply EIP20 for a better dApp ecosystem.

solidity/token-erc20.sol

@@ -16,6 +16,9 @@ contract TokenERC20 {
 
     // This generates a public event on the blockchain that will notify clients
     event Transfer(address indexed from, address indexed to, uint256 value);
+
+    // This generates a public event on the blockchain that will notify clients
+    event Approval(address indexed _owner, address indexed _spender, uint256 _value);
 
     // This notifies clients about the amount burnt
     event Burn(address indexed from, uint256 value);
@@ -97,6 +100,7 @@ contract TokenERC20 {
     function approve(address _spender, uint256 _value) public
         returns (bool success) {
         allowance[msg.sender][_spender] = _value;
+        emit Approval(msg.sender, _spender, _value);
         return true;
     }