Adding context to AsyncSpec

Allows you to do the following 
```javascript
var async = new AsyncSpec(this);
async.beforeEach(function (done) {
  doSomePromise().then(function(foo){
    this.foo = foo; 
    done();
    });
  });
async.it('should have access to foo', function () {
  expect(this.foo).toBe('bar');
});

```