Add Litecoin support to oclvanitygen

Used this commit as reference:

https://github.com/dasher/vanitygen/commit/69b66176fe9d7899922c3c6ae3f75360abdf492a#diff-2b55bd999c116ae04ed4e85e08c828abR1404

Fixes #7.
Merge pull request #8 from colindean/patch-2

Add Litecoin support to oclvanitygen