Litecoin: Adding QR Scanner Preference

You can now go into settings and pick which QR scanner app
you want to use.  This can close #17.  I installed QR Droid
and Barcode Scanner on my Nexus 7 and it works great - I can
select and scan and it works as expected.  The default is
still Barcode Scanner.  If you don't have ANY barcode scanning
apps, it will ask you to install Barcode Scanner, but it should
work fine with just QR Droid installed since it's now in the
IntentIntegrator whitelist.