use landscape view for QR scanner, add instruction overlay

Ben,

I have been using the wallet app and I love it.  Was hoping to contribute.  See below for a change to the QR screen.  I adds an overlay with instructions ( currently untranslated ) as well as superficial bounding box where poeple can center the QR code.  It doesnt affect the functionality of the scanner at all, just the presentation.  I also pulled all the logic into a QrScannerActivity to differentiate the logic from the ZBar lib.  Would love to know if you like it, whether you think its the dumbest thing ever or if you think it's workable with some changes.

-Paul

qr scanner screen with overlay
![device-2013-10-01-163342](https://f.cloud.github.com/assets/127371/1249913/98295bee-2ae9-11e3-9307-f3837538ce39.png)