Install using 'install' instead of 'cp'. Install 'todo.sh' instead of 'todo'
Allow the use of global config file

If not found elsewhere, look for the config file in /etc/todo/config.
`make install` installs a global config file, setting TODO_DIR=~/.todo.
todo.sh creates TODO_DIR if it doesn't exist.
Merge pull request #111 from mgarrido/global_opts

Allow the use of a global config file