Switch Lists to Sets for avoiding duplicates. UX Tweaks.

Use a Set instead of List for filtering duplicates.

This commit also automatically marks inputType for EmailAutoCompleteTextView as email. This displays a friendlier keyboard layout in case the user's email isn't on the device.
Also added `android:actionLabel` in the README for folks who might find it helpful.

Tips from https://plus.google.com/110199935346260350060/posts/CNkNa5N6rMz