add safe ERC20 helpers

contracts/token/SafeERC20.sol

@@ -0,0 +1,22 @@
+pragma solidity ^0.4.11;
+
+import './ERC20Basic.sol';
+import './ERC20.sol';
+
+/**
+ * @title SafeERC20
+ * @dev Wrappers around ERC20 operations that throw on failure.
+ */
+library SafeERC20 {
+  function safeTransfer(ERC20Basic token, address to, uint256 value) internal {
+    assert(token.transfer(to, value));
+  }
+
+  function safeTransferFrom(ERC20 token, address from, address to, uint256 value) internal {
+    assert(token.transferFrom(from, to, value));
+  }
+
+  function safeApprove(ERC20 token, address spender, uint256 value) internal {
+    assert(token.approve(spender, value));
+  }
+}