Add ddescribe, iit for exclusive suites,specs

When any ddescribe is registered, only specs withing these exclusive suites will be run

````javascript
describe('normal', function() {
  ddescribe('exclusive', function() {
    // all specs here will be run
  });

  // nothing here will run
  it('should not run', function() {});
});
````

When any iit is registered, only these exclusive specs will be run (precedence over ddescribe)

````javascript
it('should not run', function() {});
iit('should run', function() {});
it('should not run 2', function() {});
iit('should run as well', function() {});
````

We've been using this feature for quit a while during development of [AngularJS](http://angularjs.org).

It's part of [Jasmine - JsTD adapter](https://github.com/mhevery/jasmine-jstd-adapter), I think it was originally an idea of @IgorMinar.

I really miss this feature when using [jasmine-node](https://github.com/mhevery/jasmine-node) or when using [SlimJim](https://github.com/vojtajina/slim-jim).

I think, it really should be part of the core, so I did implement it.