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.
If it would make things more complex don't bother, I was just thinking that it's easier conceptually for me to have one file per object to return.

If you're going to have one file for both, maybe a different file name though?  The name `elementArrayFinder.js` suggests that someone might write `var ElemenetArrayFinder = require('./elementArrayFinder.js')`.  I might try something like `elementFinders.js`.