Add method to normalize card values, not used yet

Add a _normalize_card method to main_controller which accepts a
string representing a library card number, then returns that number
after normalizing it in a few ways:

* Remove all whitespace Lowercase the string If the string looks
* like a license or ID, strip all but a set number of characters

Added two TODO comments where I think it makes sense to use this
normalization.

Signed-off-by: Jeff Godin <jgodin@tadl.org>