refactor default.nix

- change layout of default.nix without changing the semantics
update readme with new nix-shell syntax
add elastic search version 1.3.4 to installation
ignore var/ but keep directories needed
split default.nix and shell.nix

- simplify the execution of nix-build and nix-shell which don't need any
  further parameters
- use a buildEnv in default.nix for nix-build
- use a derivation in shell.nix for nix-shell
simplify Makefile by using lookup through $PATH