Remove explicit code exec when required

This change allows this file to be loaded or called from other code or console.
To require this file, make sure the directory is on the load path. For
example:

$> irb -I . -r rubytapas_downloader.rb
Merge pull request #1 from jschairb/remove_implicit_exec

Remove explicit code exec when required
Provide CLI Options for username, password, and download dir

The script will look for environment variables of RTAPAS_USERNAME and
RTAPAS_PASSWORD to use in place of editing the script. Additionally, you
may pass a directory as an argument to the script that files should be
downloaded into. If the directory doesn't exist, it will be created.