Credentials improvements

Bug fixes

• Fix the problem that saves credentials file in the same repository and vpn command doesn't work in any other folder. Now the credentials file is saved in the home folder ~/.barracuda-vpn and sent to vpn-open by parameter, not hardcoded;

Improvements

• Join the user and password credential in just one file called credentials;
• Change the permission of key and credentials just to read;
• Create a new make function to show-credentials;

Documentation

• Add in the documentation how to configure the server url and added the ohmyzsh as prerequisites;