Do not use primusrun

Add option to set the primus ldpath, and use it when using the primus bridge - primusrun is no longer needed.
As discussed in https://github.com/Bumblebee-Project/bumblebee-ppa/pull/10#issuecomment-15192062

Optirun bridges now come with a function to check availability. For virtualgl, we check for vglrun and vglclient. I do not have virtualgl, so this needs to be tested.
For primus, we check whether libGL.so.1 can be found in one of the primus paths.

The default for the primus paths is currently "/usr/lib/x86_64-linux-gnu/primus:/usr/lib/i386-linux-gnu/primus". This is Debian/Ubuntu specific. Should it be extended by "/usr/lib{,32,64}/primus"?