Add support for using environment variables for credentials

Added a new Authentication::createInstance() method that will try to use the environment and fallback to .edgerc.

This method will check in order:

• AKAMAI_{SECTION}_* environment variables
• if using the default section, AKAMAI_* environment variables
• the specified (or default if none) section in .edgerc
• if not using the default section, AKAMAI_* environment variables

This new method is the recommended way for instantiating an instance.

The environment variables are:

• AKAMAI_HOST or AKAMAI_{SECTION}_HOST
• AKAMAI_CLIENT_TOKEN or AKAMAI_{SECTION}_CLIENT_TOKEN
• AKAMAI_CLIENT_SECRET or AKAMAI_{SECTION}_CLIENT_SECRET
• AKAMAI_ACCESS_TOKEN or AKAMAI_{SECTION}_ACCESS_TOKEN