Add test to validate digit.

Fixes backspace problem on Firefox.
Reorder test to disable letter input.

Test length after testing for digit validity to disable letter input on last digit.
bug fix: prevents non numeric characters in card number field when length limit is reached
Stop sending undefined values to stripe
Disable letters in cc number for Firefox

Firefox returns 0 for event keyCode on letters or numbers input.
Use charCode to detect control keys in firefox
Validate keys in the same way
Allow text selection in CVC input
Only allow digits in CVC
Merge pull request #1 from zozi/master

More fixes
Merge https://github.com/anark/angular-payments

Conflicts:
	lib/angular-payments.min.js
Merge https://github.com/alexismartin/angular-payments
Run grunt; reverts a fix, but I'm okay with that
Remove whitespace
Allow backspaces when typing in full fields