implement WBNB (#80)

wbnb/WBNB.sol

@@ -0,0 +1,62 @@
+pragma solidity ^0.4.18;
+
+contract WBNB {
+    string public name     = "Wrapped BNB";
+    string public symbol   = "WBNB";
+    uint8  public decimals = 18;
+
+    event  Approval(address indexed src, address indexed guy, uint wad);
+    event  Transfer(address indexed src, address indexed dst, uint wad);
+    event  Deposit(address indexed dst, uint wad);
+    event  Withdrawal(address indexed src, uint wad);
+
+    mapping (address => uint)                       public  balanceOf;
+    mapping (address => mapping (address => uint))  public  allowance;
+
+    function() public payable {
+        deposit();
+    }
+    function deposit() public payable {
+        balanceOf[msg.sender] += msg.value;
+        Deposit(msg.sender, msg.value);
+    }
+    function withdraw(uint wad) public {
+        require(balanceOf[msg.sender] >= wad);
+        balanceOf[msg.sender] -= wad;
+        msg.sender.transfer(wad);
+        Withdrawal(msg.sender, wad);
+    }
+
+    function totalSupply() public view returns (uint) {
+        return this.balance;
+    }
+
+    function approve(address guy, uint wad) public returns (bool) {
+        allowance[msg.sender][guy] = wad;
+        Approval(msg.sender, guy, wad);
+        return true;
+    }
+
+    function transfer(address dst, uint wad) public returns (bool) {
+        return transferFrom(msg.sender, dst, wad);
+    }
+
+    function transferFrom(address src, address dst, uint wad)
+    public
+    returns (bool)
+    {
+        require(balanceOf[src] >= wad);
+
+        if (src != msg.sender && allowance[src][msg.sender] != uint(-1)) {
+            require(allowance[src][msg.sender] >= wad);
+            allowance[src][msg.sender] -= wad;
+        }
+
+        balanceOf[src] -= wad;
+        balanceOf[dst] += wad;
+
+        Transfer(src, dst, wad);
+
+        return true;
+    }
+}