Allow chaining tests and promises.

- `.fulfilled`, `.not.rejected` and `.not.rejectedWith` when successful: the promise they return is fulfilled with the same value as the promise they were testing.
- When `.not.fulfilled`, `.rejected` and `.rejectedWith` are successful: The promise they return is fulfilled with the rejection reason.

In all cases, when they fail, the promise they return is rejected and has for reason the assertion that failed.

The changes above make it so that the `object` flag is automatically set when the promise returned is resolved, and thus also allows chaining.

This fixes #113.

lib/chai-as-promised.js

@@ -105,7 +105,6 @@
             var that = this;
             var derivedPromise = getBasePromise(that).then(
                 function (value) {
-                    that._obj = value;
                     assertIfNegated(that,
                                     "expected promise not to be fulfilled but it was fulfilled with #{act}",
                                     { actual: value });
@@ -115,6 +114,7 @@
                     assertIfNotNegated(that,
                                        "expected promise to be fulfilled but it was rejected with #{act}",
                                        { actual: reason });
+                    return reason;
                 }
             );
 
@@ -125,7 +125,6 @@
             var that = this;
             var derivedPromise = getBasePromise(that).then(
                 function (value) {
-                    that._obj = value;
                     assertIfNotNegated(that,
                                        "expected promise to be rejected but it was fulfilled with #{act}",
                                        { actual: value });
@@ -183,9 +182,8 @@
                         expected = desiredReason;
                     }
 
-                    that._obj = value;
-
                     assertIfNotNegated(that, assertionMessage, { expected: expected, actual: value });
+                    return value;
                 },
                 function (reason) {
                     if (Constructor) {
@@ -223,6 +221,7 @@
                                     desiredReason,
                                     reason);
                     }
+                    return reason;
                 }
             );