Env vars for twilio info

[kecolus]

Credentials and other sensitive information can be hard coded into an application, stored in configuration files, or stored in environment variables. Using environment variables reduces the danger of someone accidentally publishing their credentials.

[kecolus]

Because I updated my master branch before creating this change, this pull request also includes the contents of https://github.com/neil-lobracco/twilio-rs/pull/3. Sorry about the confusion.

[lancecarlson]

@kecolus I feel like introducing an env variable requirement should be something the end user decides to add or not? I think this more of an application level decision, not a library.