feat(element): allow chaining of element finders with element().element()...

Chaining calls to element will now build a scoped element finder. No webdriver
functions will be called until a method (such as getText) is called on the
final element. Example:

    var elem = element(by.id('outer')).element(by.css('inner'));
    browser.get('myPage');
    elem.click();

Closes #340.