feat(protractor): allow advanced features for ElementArrayFinder

changed ElementFinder as a subset of an ElementArrayFinder
There are no breaking changes. Everything that works before _should_ work the same way now (same syntax and all). 

Features:
* Finding through multiple branches instead of one:
```
@view
<div id='id1' class="parent">
  <ul>
    <li class="foo">1a</li>
    <li class="baz">1b</li>
  </ul>
</div>
<div id='id2' class="parent">
  <ul>
    <li class="foo">2a</li>
    <li class="bar">2b</li>
  </ul>
</div>

@example
var foo = element.all(by.css('.parent')).all(by.css('.foo'))
expect(foo.getText()).toEqual(['1a', '2a'])
var baz = element.all(by.css('.parent')).all(by.css('.baz'))
expect(baz.getText()).toEqual(['1b'])
var nonexistent = element.all(by.css('.parent')).all(by.css('.NONEXISTENT'))
expect(nonexistent.getText()).toEqual([''])
```

* Filters can be chained and returns ElementArrayFinder 
* ElementArrayFinder and ElementFinder are integrated together
```
i.e.
element.all(by.W).all(by.X).filter(filterFn1).filter(filterFn2).last().element(by.Y).all(by.Z)
```

* Actions can be done on an ElementArrayFinder
```
    var multiElement = $$('#checkboxes').filter(function(elem, index) {
      return elem.getText().then(function(text) {
        return text === 'checkbox1' || text === 'checkbox2';
      });
    });
    multiElement.click();
    //expect: both checkboxes should be checked;
```

All of the above features (i.e. filter, get/first/last, all, element) can be incorporated into page objects. Especially with the filter, you should be able to build much stronger selectors for your page objects. 

Lastly, your test should run _slightly_ faster now in theory because of the implementation changes (and protractor's tests did go from 113.817s to 106.551s), but this is not a guarantee and is just a side effect
We never got rid of browser.findElement, so it should still be the same. Here's the full definition:

```
Protractor.prototype.findElement = function(locator) {
  return this.element(locator).getWebElement();
};
```