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.
Merge pull request #2 from jschairb/cli_options

Provide CLI Options for username, password, and download dir