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.
People want to extend it (https://github.com/angular/protractor/issues/1584). I think it's a reasonable request for people to build on top of protractor (applitools is another example)