Adding per gpu --gpu-threads support

I have a 2 * R9 280X and a 7850 Card in the same rig.. So I've needed to use -g2 in the first 2 and -g1 in the last one.. Cgminer had a bug with this parameter that only allowed a global -g parameter (same for all cards)

So, I've modified vertminer to add the changes from a cgminer 3.7.2 fork. Here is the original diff

https://github.com/Kalroth/cgminer-3.7.2-kalroth/commit/5a4d75b3e8ac7feeb1c9dceb8ca4615dacda4178


I've builded this in Centos 6.5 and works perfectly. So, this is my contribution.
![vertminer](https://f.cloud.github.com/assets/333676/2479712/b5ab7a02-b0a0-11e3-9f09-0dd3f720805a.png)