Add small script to setup label in lesson repo

The script make easy to create the labels and keep it compatible
across repositories.
Improve script to setup labels

Address @gvwilson comments:

- add an example
- variables in the shell are UPPER_CASED
- Check that repository exists using a HEAD
Avoid JSON outup from setup-labels

As requested by @gvwilson, don't print the JSON response from
GitHub.

Others changes:

- Use arrays to store the label and its color
- Print message if fail
Merge pull request #18 from r-gaia-cs/add-script-for-label

Add small script to setup label in lesson repo