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.

src/core/Env.js

@@ -296,6 +296,18 @@ getJasmineRequireObj().Env = function(j$) {
       }
     };
 
+    this.addCustomObjectFormatter = function(formatter) {
+      if (!currentRunnable()) {
+        throw new Error(
+          'Custom object formatters must be added in a before function or a spec'
+        );
+      }
+
+      runnableResources[currentRunnable().id].customObjectFormatters.push(
+        formatter
+      );
+    };
+
     j$.Expectation.addCoreMatchers(j$.matchers);
     j$.Expectation.addAsyncCoreMatchers(j$.asyncMatchers);
 
@@ -310,7 +322,9 @@ getJasmineRequireObj().Env = function(j$) {
     };
 
     var makePrettyPrinter = function() {
-      return j$.makePrettyPrinter();
+      var customObjectFormatters =
+        runnableResources[currentRunnable().id].customObjectFormatters;
+      return j$.makePrettyPrinter(customObjectFormatters);
     };
 
     var makeMatchersUtil = function() {
@@ -360,7 +374,8 @@ getJasmineRequireObj().Env = function(j$) {
         customMatchers: {},
         customAsyncMatchers: {},
         customSpyStrategies: {},
-        defaultStrategyFn: undefined
+        defaultStrategyFn: undefined,
+        customObjectFormatters: []
       };
 
       if (runnableResources[parentRunnableId]) {