Add ReentrancyGuard

contracts/ReentrancyGuard.sol

@@ -0,0 +1,28 @@
+pragma solidity ^0.4.8;
+
+/// @title Helps contracts guard agains rentrancy attacks.
+/// @author Remco Bloemen <remco@2π.com>
+/// @notice If you mark a function `nonReentrant`, you should also
+/// mark it `external`.
+contract ReentrancyGuard {
+
+  /// @dev We use a single lock for the whole contract.
+  bool private rentrancy_lock = false;
+
+  /// Prevent contract from calling itself, directly or indirectly.
+  /// @notice If you mark a function `nonReentrant`, you should also
+  /// mark it `external`. Calling one nonReentrant function from
+  /// another is not supported. Instead, you can implement a
+  /// `private` function doing the actual work, and a `external`
+  /// wrapper marked as `nonReentrant`.
+  modifier nonReentrant() {
+    if(rentrancy_lock == false) {
+      rentrancy_lock = true;
+      _;
+      rentrancy_lock = false;
+    } else {
+      throw;
+    }
+  }
+
+}