Adding proxy support to SDK

Added [:proxy] hash to Faraday connection. Tested using a local proxy (Fiddler). Host is required, the remaining settings (port, user & password) are optional.

To use in a plugin, add the following to newrelic_plugin.yml in the `newrelic:` stanza:

```
newrelic:
  # 
  #
  # Proxy Settings - Uncomment and set if using a proxy.
  # proxy_host is required
  # proxy_port, proxy_user & proxy_pass are all optional
  #
  proxy_host: yourhost.com
  proxy_port: 8888
  proxy_user: seth
  proxy_pass: redacted
```