Fix for issue #90 (Make install)

* Make install: copies the script (to /usr/local/bin) and the completions file, like danohuiginn did in #90. In addition, it creates the config file /etc/todo/config. This file sets TODO_DIR=~/.todo.

* If not found elsewhere, todo.sh looks for the config file in /etc/todo/config.

* todo.sh tries to create $TODO_DIR if it doesn't exists.