chore(elementArrayFinder): separate elementFinder into its own file

protractor.js is growing very big. This separates ElementFinder/ElementArrayFinder (which really don't belong to the protractor class) into its own file. 
Also helps with unit testing in the future by injecting a mock ptor object.
Let's keep it in one file (they are tightly bound) and call it `element.js` (matches what the test file is called, encompasses both)