Add OAuthService

[richardm-stripe]

r? @ob-stripe
cc @stripe/api-libraries

Fixes #958 and adds $stripe->oauth with authorizeUrl, token, and deauthorize methods.

I chose to introduce ->requestConnect into the StripeClientInterface, rather than overload ->request, because to my intuition this seems cleaner, but I'm interested in second opinions.

[ob-stripe]

I would prefer if the requestConnect stuff was kept inside OAuthService, since it's OAuth specific and very unlikely to be used by any other services in future.

[richardm-stripe]

ptal @ob-stripe -- I've addressed all your comments & moved 'connectRequest' out into OAuthService. Thanks for the careful review.

[ob-stripe]

A few more comments, but this is looking great already

[richardm-stripe]

ptal @ob-stripe

1. Removed unnecessary calls to ->buildPath for constant paths.
2. Changed $defaultOptions back to private.
3. Changed ->authorizeUrl to use api_base instead of connect_base -- I refactored this into $opts->apiBase = $this->_getBase() function.
4. I made ->_getBase() throw an exception if connect_base is passed. Might save a user a few cycles when switching from \Stripe\OAuth.

[ob-stripe]

I'm not sure this will work. \Stripe\RequestOptions does not have a connect_base property, even if there is a connect_base key in the array passed to RequestOptions::parse(), so I think this condition will always be false. Mind adding a test for it?

[richardm-stripe]

Oops! Good catch. Fixed now.

[richardm-stripe]

ptal @ob-stripe fixed the $opts['connect_base'] check and added docblocks.

[ob-stripe]

LGTM!