cudatoolkit: generalize derivations, add cuda7

There is not much different between the cudatoolkit
expressions:

  pkgs/development/compilers/cudatoolkit/5.5.nix
  pkgs/development/compilers/cudatoolkit/6.0.nix
  pkgs/development/compilers/cudatoolkit/6.5.nix

This commit removes those and replaces them with

  pkgs/development/compilers/cudatoolkit/generic.nix

and adds cudatoolkit version 7