Put CLI and Client under a common Aeolus module

These module names were changed:

AeolusCli    -> Aeolus::Cli
AeolusClient -> Aeolus::Client

This scopes our code under common Aeolus module.

To adhere to gem naming conventions in Ruby, the gem name was changed
accordingly:

aeolus_cli -> aeolus-cli

Closes #20.