Gem

Letteropend is now a gem! It exposes a `Letteropend` library, and has a `letteropend` CLI.

- Fixes #1.
- `get_film_urls.rb` and `get_total_minutes.rb` are now merged into `letteropend.rb`, which also exposes a `Letteropend` module with these methods.
- The example scripts are no longer standalone, and are now three subcommands in a new `CLI` module.
- Examples no longer take STDIN, and instead read command line arguments.
- The MIT license is now used.

If you decide to merge this, please double check `letteropend.gemspec` and update the metadata if necessary.