added redeemWalletCode method

[Rob--]

I have not actually tested this method on a valid Steam code yet. My assumption is that if the wallet code is valid the credit will be redeemed to the user's Steam account without further interaction required.

Edit: Usage will look like this:

store.redeemWalletCode('RWNFQ-90K07-MP82Q', function(err, result, redeemed){
        // result will look like: { EResult: 1, EPurchaseResult: 15 }
        // so if `redeemed` is false, then they must look at the result parameter to understand why the request failed
})

[DoctorMcKay]

Good thing I went and bought a card to test this with. The confirmredeemwalletcode request is required; it's what actually redeems the code. validatewalletcode is merely a pre-check.

[Rob--]

Should've done that, sorry. It's just that from this discussion about redeeming wallet codes I saw this comment and this commit and it didn't seem like a further request was required to redeem the code.