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 chaijs#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;
                 }
             );