Add support for fingerprints

[AlstonLin]

Although it basically makes the main purpose of the app pointless (no ML), it makes it so people can still use the lock screen and still benefit from a fingerprint scanner if they have one.

See http://stackoverflow.com/questions/34807574/implementing-fingerprint-authentication-for-android

[AlstonLin]

Not that important since people with a fingerprint scanner wont really need this app