Add a constant ‘nixpkgsVersion’

This makes it a bit easier to find out the current Nixpkgs version,
e.g.

  $ nix-instantiate --eval '<nixpkgs>' -A lib.nixpkgsVersion
  "14.02.35657.66f51a9"
Add a global variable ‘inNixShell’

This is primarily useful in Nix expressions that are also intended to
be used by nix-shell.  It allows saying things like:

  buildInputs = [ ... ] ++ (if inNixShell then [ emacs ] else [ ]);

This should not be used in Nixpkgs (since the question of what
constitutes a useful nix-shell environment is very subjective).