[#179] - Changed payment amount to number_to_currency

In index:
![image](https://cloud.githubusercontent.com/assets/6850632/2970884/d2ab39de-db62-11e3-9a22-6099d8a21a66.png)
In show:
![image](https://cloud.githubusercontent.com/assets/6850632/2970889/e984c5f8-db62-11e3-8799-6c7de3f2179d.png)