Make Log a method of Client

[jmfederico]

This a feature request.

Quite happy with Rollbar and its Go library, but we do miss the option to have rollbar.Log available to use with any client and not only the default one.

As it is right now we have to wrap rollbar's clients in our own struct that implements a copy of rollbar.Log to emulate the behaviour around requests and error attributes.

This would be ideal:

import "github.com/rollbar/rollbar-go"

rb_client := rollbar.New(...)
// stuff happens
rb_client.Log(error, request, ...)