Return container from Fixture.set() & setFixture()

We have added some syntax sugar which makes it easier to scope test selections to the fixture container. We can now do:

```javascript
var fixture = setFixture('<div class="foo">foo</div>')
var foo = fixture.find('.foo')
```

Instead of:
```javascript
setFixture('<div class="foo">foo</div>')
var foo = $('#' + jasmine.getFixture().containerId + ' .foo')
```
Or:
```javascript
setFixture('<div class="foo">foo</div>')
var foo = $('.foo') //could conflict
```