Add to Exchange features imported from DexTrade (#20)

* Add code from abandoned Trade smart contract

* Add initial unit test

contracts/Exchange.sol

@@ -10,14 +10,43 @@ pragma solidity ^0.4.21;
 /**
 @title Exchange
 @notice The orchestrator of trades and payments on-chain.
-@dev The essence and trade specific parts of the previous Trade smart contract.
 */
 contract Exchange {
 
+    //
+    // Structures
+    // -----------------------------------------------------------------------------------------------------------------
+    struct Signature {
+        bytes32 r;
+        bytes32 s;
+        uint8 v;
+    }
+
+    struct Trade {
+        uint256 buyOrderHash;
+        uint256 sellOrderHash;
+        uint256 buyerOrderNonce;    // These ones should be obtained from orders map (accessed by hash),
+        uint256 sellerOrderNonce;   // but we don't have such information.
+        address buyer;
+        address seller;
+        uint256 tokenAmount;
+        uint256 etherAmount;
+        address token;
+        bool immediateSettlement;
+        Signature signature;
+    }
+
+    struct Settlement {
+        // uint256[] tradeNonces;
+        // uint256[] tradeHashes;
+        // // Balances here
+        // uint256[] signature;
+    }
+
     //
     // Variables
     // -----------------------------------------------------------------------------------------------------------------
-    address private owner;
+    address public owner;
 
     //
     // Events

test/scenarios/Exchange.js

@@ -1,7 +1,53 @@
-module.exports = function (glob) {
-	describe("Exchange", function () {
-		it("T001: TO-DO", function (done) {
-			done();
-		});
-	});
+const chai = require('chai');
+const chaiAsPromised = require("chai-as-promised");
+const Exchange = artifacts.require('Exchange');
+
+chai.use(chaiAsPromised);
+chai.should();
+
+module.exports = (glob) => {
+    describe('Exchange', () => {
+
+        let instance = null;
+
+        before(async () => {
+            instance = await Exchange.deployed();
+        });
+
+        describe('constructor', () => {
+            it('should initialize fields', async () => {
+                const owner = await instance.owner.call();
+                owner.should.equal(glob.owner);
+            });
+        });
+
+        describe('owner()', () => {
+            it('should equal value initialized at construction time', async () => {
+                const owner = await instance.owner.call();
+                owner.should.equal(glob.owner);
+            });
+        });
+
+        describe('changeOwner()', () => {
+            describe('if called with current owner as sender', () => {
+                after(async () => {
+                    await instance.changeOwner(glob.owner, {from: glob.user_a});
+                });
+
+                it('should successfully set new owner and emit event', async () => {
+                    const result = await instance.changeOwner(glob.user_a);
+                    result.logs.should.be.an('array').and.have.lengthOf(1);
+                    result.logs[0].event.should.equal('OwnerChangedEvent');
+                    const owner = await instance.owner.call();
+                    owner.should.equal(glob.user_a);
+                });
+            });
+
+            describe('if called with sender that is not current owner', () => {
+                it('should fail to set new owner', async () => {
+                    instance.changeOwner(glob.user_a, {from: glob.user_a}).should.be.rejected;
+                });
+            });
+        });
+    });
 };