Consider 'callback_url' optional at __construct()

[vdyalex]

Hi @ashwinks,

I've added support to the callback_url parameter at config array to be optional.

When using the library, I've extended it on my projects for reuse with multiple providers and what I would like to avoid is to set the callback_url on every instantiation. It's only required to authentication flow, as the same way in other OAuth2 APIs.

Could you merge it into your repository? The changes you not affect who is using the library. It's a stable update.

Thanks, Alex