added function to renounce ownership (#907)

contracts/ownership/Ownable.sol

@@ -10,6 +10,7 @@ contract Ownable {
   address public owner;
 
 
+  event OwnershipRenounced(address indexed previousOwner);
   event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
 
 
@@ -39,4 +40,11 @@ contract Ownable {
     owner = newOwner;
   }
 
+  /**
+   * @dev Allows the current owner to relinquish control of the contract.
+   */
+  function renounceOwnership() public onlyOwner {
+    emit OwnershipRenounced(owner);
+    owner = address(0);
+  }
 }

test/ownership/Ownable.test.js

@@ -2,6 +2,7 @@
 import assertRevert from '../helpers/assertRevert';
 
 var Ownable = artifacts.require('Ownable');
+const ZERO_ADDRESS = '0x0000000000000000000000000000000000000000';
 
 contract('Ownable', function (accounts) {
   let ownable;
@@ -34,4 +35,18 @@ contract('Ownable', function (accounts) {
     let originalOwner = await ownable.owner();
     await assertRevert(ownable.transferOwnership(null, { from: originalOwner }));
   });
+
+  it('loses owner after renouncement', async function () {
+    await ownable.renounceOwnership();
+    let owner = await ownable.owner();
+
+    assert.isTrue(owner === ZERO_ADDRESS);
+  });
+
+  it('should prevent non-owners from renouncement', async function () {
+    const other = accounts[2];
+    const owner = await ownable.owner.call();
+    assert.isTrue(owner !== other);
+    await assertRevert(ownable.renounceOwnership({ from: other }));
+  });
 });