Added function for user to specify which cracking machine to use

![screenshot from 2013-11-11 22-46-41](https://f.cloud.github.com/assets/5349388/1519066/7b11c01c-4b4d-11e3-9a0c-611756e6c6c5.png)

Just super-tiny change.
Want to know how pull request work.
:)