changed "card" endpoint to "cards", to properly match Stripe API

Also added in defaults if a call to cards.list was missing count and
offset (10 and 0 respectively)

Updated readme to reflect changes
updated library to only send count and offset if they are truthy (previously, we sent them irregardless of what was received).

Updated readme to include a note about optional values.
Merge pull request #1 from julianlam/master

- changed "card" endpoint to "cards", to properly match Stripe API
- patched nArgs to populate count and offset for list() functions correctly