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/matchers/DiffBuilderSpec.js

@@ -44,4 +44,30 @@ describe("DiffBuilder", function() {
 
     expect(diffBuilder.getMessage()).toEqual("I find your lack of foo disturbing. (was bar, at $.x)");
   });
+
+  it("uses the injected pretty-printer", function() {
+    var prettyPrinter = function(val) {
+        return '|' + val + '|';
+      },
+      diffBuilder = jasmineUnderTest.DiffBuilder({prettyPrinter: prettyPrinter});
+
+    diffBuilder.withPath('foo', function() {
+      diffBuilder.record('actual', 'expected');
+    });
+
+    expect(diffBuilder.getMessage()).toEqual("Expected $.foo = |actual| to equal |expected|.");
+  });
+
+
+  it("passes the injected pretty-printer to the diff formatter", function() {
+    var diffFormatter = jasmine.createSpy('diffFormatter'),
+      prettyPrinter = function() {},
+      diffBuilder = jasmineUnderTest.DiffBuilder({prettyPrinter: prettyPrinter});
+
+    diffBuilder.withPath('x', function() {
+      diffBuilder.record('bar', 'foo', diffFormatter);
+    });
+
+    expect(diffFormatter).toHaveBeenCalledWith('bar', 'foo', jasmine.anything(), prettyPrinter);
+  });
 });