Added support for custom object formatters

Custom object formatters allow users to customize how an object is
stringified in matcher failure messages. This can already be done by
adding a `jasmineToString` method to the objects in question. But
it's not always desirable or possible to do that, particularly when
objects of a given "type" do not inherit from a specific prototype.
For instance, suppose a web service returns a list of foos that are
deserialized from JSON, e.g.:

   { fooId: 42, /* more properties */ }

The only way to define `jasmineToString` on those is by writing code to
add it to each instance at runtime. But a custom object formatter can
recognize that the object it's looking at is a foo and format it
accordingly:

   jasmine.addCustomObjectFormatter(function(obj) {
      if (typeof obj.fooId !== 'number') {
            return undefined;
        }

        return '[Foo with ID ' + obj.fooId + ']';
    });

Unlike `jasmineToString`, custom object formatters are scoped to a
particular spec or suite and don't require any changes to the code
under test.

spec/core/integration/CustomObjectFormatterSpec.js

@@ -0,0 +1,67 @@
+describe("Custom object formatters", function() {
+  var env;
+
+  beforeEach(function() {
+    env = new jasmineUnderTest.Env();
+    env.configure({random: false});
+  });
+
+  it("scopes custom object formatters to a spec", function(done) {
+    env.it('a spec with custom pretty-printer', function() {
+      env.addCustomObjectFormatter(function(obj) { return 'custom(' + obj + ')'; });
+      env.expect(42).toBeUndefined();
+    });
+
+    env.it('a spec without custom pretty-printer', function() {
+      env.expect(42).toBeUndefined();
+    });
+
+    var specResults = [];
+    var specDone = function(result) {
+      specResults.push(result);
+    };
+    var expectations = function() {
+      expect(specResults[0].failedExpectations[0].message).toEqual("Expected custom(42) to be undefined.");
+      expect(specResults[1].failedExpectations[0].message).toEqual("Expected 42 to be undefined.");
+      done();
+    };
+    env.addReporter({ specDone:specDone, jasmineDone: expectations});
+
+    env.execute();
+  });
+
+  it("scopes custom object formatters to a suite", function(done) {
+    env.it('a spec without custom pretty-printer', function() {
+      env.expect(42).toBeUndefined();
+    });
+
+    env.describe('with custom pretty-printer', function() {
+      env.beforeEach(function() {
+        env.addCustomObjectFormatter(function(obj) { return 'custom(' + obj + ')'; });
+      });
+
+      env.it('a spec', function() {
+        env.expect(42).toBeUndefined();
+      });
+    });
+
+    var specResults = [];
+    var specDone = function(result) {
+      specResults.push(result);
+    };
+    var expectations = function() {
+      expect(specResults[0].failedExpectations[0].message).toEqual("Expected 42 to be undefined.");
+      expect(specResults[1].failedExpectations[0].message).toEqual("Expected custom(42) to be undefined.");
+      done();
+    };
+    env.addReporter({ specDone:specDone, jasmineDone: expectations});
+
+    env.execute();
+  });
+
+  it("throws an exception if you try to add a custom object formatter outside a runable", function() {
+    expect(function() {
+      env.addCustomObjectFormatter(function() {});
+    }).toThrowError('Custom object formatters must be added in a before function or a spec')
+  });
+});