Validate digit and reorder test.

[alexismartin]

Fixes issue #49 and #51.

[tjwallace]

+1

[jamesbrobb]

+1

[jamesbrobb]

@alexismartin i've noticed that there's also a (mildly annoying but not fatal) issue when deleting the expiry date in firefox. The backslash is deleted on key down, but then re-added on press by the _formatForwardExpiry handler method

line 333 needs to be changed from

if (/^\d\d$/.test(val)) {

to

if (/^\d\d$/.test(val) && /^\d+$/.test(digit)) {

That said, i can't actually see the purpose of the _formatForwardExpiry handler method, as the existing conditional logic only equates to true when the delete key is being pressed. Otherwise _formatExpiry deals with adding the forward slash as the user types.