Display banner notification about impending credit card expiration. Resolves #2211
Added caching requests to Balanced
add mappings to StripeCard for expiration attributes

https://stripe.com/docs/api/python#cards
make quotes style consistent
fix type of `NOTIFIED_ABOUT_EXPIRATION`
Merge pull request #2379 from gittip/expiration-banner

Display banner notification about credit card expiration