Adding new Superuser contract with test (#952)

* Adding new Superuser contract + tests

contracts/ownership/Superuser.sol

@@ -0,0 +1,62 @@
+pragma solidity ^0.4.23;
+
+
+import "./Ownable.sol";
+import "./rbac/RBAC.sol";
+
+
+/**
+ * @title Superuser
+ * @dev The Superuser contract defines a single superuser who can transfer the ownership 
+ * @dev of a contract to a new address, even if he is not the owner. 
+ * @dev A superuser can transfer his role to a new address. 
+ */
+contract Superuser is Ownable, RBAC {
+  string public constant ROLE_SUPERUSER = "superuser";
+
+  constructor () public {
+    addRole(msg.sender, ROLE_SUPERUSER);
+  }
+
+  /**
+   * @dev Throws if called by any account that's not a superuser.
+   */
+  modifier onlySuperuser() {
+    checkRole(msg.sender, ROLE_SUPERUSER);
+    _;
+  }
+
+  /**
+   * @dev getter to determine if address has superuser role
+   */
+  function isSuperuser(address _addr)
+    public
+    view
+    returns (bool)
+  {
+    return hasRole(_addr, ROLE_SUPERUSER);
+  }
+
+  /**
+   * @dev Allows the current superuser to transfer his role to a newSuperuser.
+   * @param _newSuperuser The address to transfer ownership to.
+   */
+  function transferSuperuser(address _newSuperuser) 
+    onlySuperuser
+    public
+  {
+    require(_newSuperuser != address(0));
+    removeRole(msg.sender, ROLE_SUPERUSER);
+    addRole(_newSuperuser, ROLE_SUPERUSER);
+  }
+
+  /**
+   * @dev Allows the current superuser to transfer control of the contract to a newOwner.
+   * @param _newOwner The address to transfer ownership to.
+   */
+  function transferOwnership(address _newOwner) public onlySuperuser {
+    require(_newOwner != address(0));
+    owner = _newOwner;
+    emit OwnershipTransferred(owner, _newOwner);
+  }
+}