Webpay: modify currency unit from yen (1) to sen (1/100) in JPY

Current payment gateways which support non fractional currencies including JPY such as PayPal accept JPY payment as below.

``` transaction_in_sen.rb
gateway.purchase(10000, credit_card) # for charging 100 JPY
```

In this case, the currency unit is sen (0.01 yen).

On the other hand, WebpayGateway handle the amount as below and the currency unit is yen. 

``` transaction_in_yen.rb
gateway.purchase(100, credit_card) # for charging 100 JPY
```

In this pull request, I modify WebpayGateway to use sen (0.01 yen) as the currency unit like the first example to remove possible confusion.

This modification is requested originally from @louiskearns