Correct amex cvc validation

### Correct amex cvc validation

I understand this could be an area of contention or debate, however:

* I did some research online and found that the 3 digit security code for AMEX is generally used only for internal support requests between a customer and AMEX, and I believe the predominant use-case of this library (developers who accept payments online) is best served with a three digit amex cvc being invalid. Take a look at [this forum discussion regarding the matter](http://creditcardforum.com/american-express/5148-amex-3-digit-security-code.html).
* The README states that three digit amex cvc's will be invalid as well, and they currently are not. 
* Feel free to do your own searching on the [American Express help website](https://search.americanexpress.com/app/answers/list/search/1/kw/).
* This pull request also incorporates some more automated tests which are just good for the library to have in general. 
* Everything that is advertised in the README is tested against, and now passes, and a couple additional tests were added as well.