Release/v2.0.0

- Dial returns an interface, not a concrete type (https://github.com/sendgrid/go-statsdclient/pull/5)
- Added MockClient type for testing (https://github.com/sendgrid/go-statsdclient/pull/5)
- Removed MakePrefix method since Ops says the entire prefix should be passed in via config, not constructed by the process (https://github.com/sendgrid/go-statsdclient/pull/7)