Improved the BIC validator according to ISO 9362

And this doesn't handle all cases as it doesn't provide for lowercase characters.
http://snipplr.com/view/15320/bic-bank-identifier-code-regex/

BTW the last ? should still be present.

@johmanx10 It is, but for readability I recommend the way it is written here.
4 letters bank code
2 letters country code
That is why it written like this.