chore(elementFinder): keep a reference to the original locator

There are some reasons to expose the reference to the original webdriver.Locator, @Droogans for example [shows one here](http://stackoverflow.com/a/23567136/511069)

I personally need this because sometimes i find myself using webdriver directly, i.e. `browser.driver.xxxx` and for that reason i'm unable to use certain shortcuts like `$('a.thing')` within my page objects, using `by.css('a.thing')` then wrapping the locator with `element()` when needed.

Being able to do `element(aThing).locator()` will give the flexibility back.