Add Q.noConflict function

Fixes kriskowal#677

This commit adds a function that is basically a noop. It simply alerts the user that they should not be using `Q.noConflict` when they are not using Q as a global.

If Q finds itself sourced as a `<script>` it will store the original value of `Q` and overwites `Q.noConflict` function with an actual implementation. `Q.noConflict` returns a refernce to `Q`.

q.js

@@ -57,7 +57,18 @@
 
     // <script>
     } else {
-        Q = definition();
+        // Get the `window` object, save the previous Q global
+        // and initialize Q as a global.
+        var root = this;
+        var previousQ = root.Q;
+        root.Q = definition();
+
+        // Add a noConflict object so Q can be removed from the
+        // global namespace.
+        root.Q.noConflict = function() {
+            root.Q = previousQ;
+            return this;
+        };
     }
 
 })(function () {
@@ -1900,6 +1911,10 @@ Promise.prototype.nodeify = function (nodeback) {
     }
 };
 
+Q.noConflict = function() {
+    console.log('Q.noConflict only works when Q is used as a global');
+};
+
 // All code before this point will be filtered from stack traces.
 var qEndingLine = captureLine();
 

spec/q-spec.js

@@ -2169,6 +2169,27 @@ describe("node support", function () {
 
 });
 
+describe("browser support", function () {
+    var _Q;
+
+    beforeEach(function() {
+        _Q = Q;
+    });
+
+    afterEach(function() {
+        Q = _Q;
+    });
+
+    it("sets the global Q object to its original value", function() {
+        Q.noConflict();
+
+        // In this context the original value of Q is undefined.
+        if(typeof window === 'object') {
+            expect(Q).toEqual(undefined);
+        }
+    });
+});
+
 describe("isPromise", function () {
     it("returns true if passed a promise", function () {
         expect(Q.isPromise(Q(10))).toBe(true);