AmEx cards actually start with 34 or 37

Here's one you can try: 340000000000009
Merge pull request #55 from shezi/patch-2

AmEx cards actually start with 34 or 37
Fixes bootstrap 2.3 issue noted by @shezi on #54; potentially addresses #32 as well.
Updated README to include note about submitting compiled SCSS and CS.
adding generic style so non-specific cards can fall back to that (like the JCB card, for example)
adding Carte Bleue VISA variant