Fix Torch Install
- Distable CUDA qtlua
- Force customized luac to be named as torch-luac, the luac exist check
  for some reason does not work. Using the same luac name as lua is not
    a good naming anyway
- add build folder to gitignore
- change FindLua to FindLua51. This is a hard code, probably will break
  installation on osx and other linux distribution.