Feature/expiry

Implementation for automatically scanning the expiration date.

Currently a work in progress, not yet ready to be merged.

Currently restricted to 64-bit processors (due to an as-yet unresolved build problem for 32-bit processors), tested only with iOS, not Android.

Does a fairly good job with some credit cards, but fails completely on others.

To build this branch with expiry-scanning enabled, define the preprocessor macro SCAN_EXPIRY=1.