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.
No, it could be two files. Just figured they're so tightly bound, it's easier to put them in one file.