Fixes overflow error by adding 'using bigint;'

![example2](https://cloud.githubusercontent.com/assets/7541328/3928628/766daec4-240d-11e4-9df0-77744c991e7e.png)
![example1](https://cloud.githubusercontent.com/assets/7541328/3928629/78592042-240d-11e4-9f11-53a4fd11e029.png)
http://www.wolframalpha.com/input/?i=18442240474082181119+to+hex
http://www.wolframalpha.com/input/?i=999999999999999999+to+hex

Fixes #484. This is totally my first PR so if I need to do something else lemme know.