Some attempted performance improvements

Made BigInteger more memory efficient, this should improve PDF417 scan
times on android/ios/wp7+ or other devices with contstrained ram.

In the ZXing surface view, Run the actual detection in the background,
and skip frames while it's running. This should improve UI
Responsiveness.

And finally ,added code to auto-detect supported preview sizes, and
attempt to pick a reasonable size to improve scanning success rates.