Adding recipe for rtags, c/c++ tags package

- Requires clang to be installed on the computer
- Requires the user to be running a specific daemon
  which is built with the :build() commands provided
  in the recipe

Output of el-get-check-recipe:
/home/lefteris/.emacs.d/el-get/el-get/recipes/rtags.rcp: 0 error(s) found.