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/asymmetric_equality/ArrayWithExactContentsSpec.js

@@ -30,9 +30,14 @@ describe("ArrayWithExactContents", function() {
   });
 
   it("jasmineToStrings itself", function() {
-    var matcher = new jasmineUnderTest.ArrayWithExactContents([]);
+    var sample = [],
+      matcher = new jasmineUnderTest.ArrayWithExactContents(sample),
+      pp = jasmine.createSpy('pp').and.returnValue('sample');
 
-    expect(matcher.jasmineToString()).toMatch("<jasmine.arrayWithExactContents");
+    expect(matcher.jasmineToString(pp)).toEqual(
+      '<jasmine.arrayWithExactContents(sample)>'
+    );
+    expect(pp).toHaveBeenCalledWith(sample);
   });
 
   it("uses custom equality testers", function() {