client.go
func ClientFromEnv(key string) (*Client, error) {
	url := os.Getenv(key)
	return ClientFromURL(url)
}
func ClientFromEnv(key string) (*Client, error) {
	url := os.Getenv(key)
	return ClientFromURL(url)
}
/*
Alternatively, you can instantiate a client from an environment variable.
This is particularly relevant if you are using Pusher as a Heroku add-on,
which stores credentials in a `"PUSHER_URL"` environment variable. For example:
	client := pusher.ClientFromEnv("PUSHER_URL")

*/