Nested ng-app compatibility

When using an angular app that exists on a descendant element of document.body, the protractor.waitForAngular function fails because it's hard coded to extract angular's injector from document.body.

```
angular.element(document.body).injector().get('$browser')
```
Fails with: *javascript error: Cannot call method 'get' of undefined*

I added a config option, targetEl, which uses an optional selector to locate the angular app in these situations.

```
targetEl: 'div#my-app-here'
```