v2.1.0

• updated and improved README
• secret_key refactored to api_key to align with docs and portal
• api_host is now known as rest_endpoint to align with docs and portal
• fixed python3.x bug with bytes vs str in verify_telesign_callback_signature
• various doc string updates and fixes
• removed requests specific config, subclass and customize instead
• added travis CI, codecov coverage and additional unit tests