Add fetchbower function to go along with bower2nix

(cherry picked from commit e6dae5d7f43087c78bdc5569b36da65207fd057e)

Signed-off-by: Shea Levy <shea@shealevy.com>

Conflicts:
	pkgs/top-level/node-packages-generated.nix
	pkgs/top-level/node-packages.json