Added OAuth2 authorization support

This is intended to be fully backwards-compatible with existing Clients and require minimal changes to transition to OAuth2.

* Created `OAuth2Authenticator : IAuthenticator`, which adds the required "Authorization : Bearer <AccessToken>" header to Dropbox requests
* Added `Client.ctor()`, `.BuilAuthorizationUrl()`, and `.GetAccessToken*()` overloads that support the OAuth2 authentication flow.
* Refactored out `Client.ctor()`, `.GetAccessToken*()` and `.GetAccountInfo*()` code that appeared to duplicate effort for setting `IRestClient.BaseUrl` and `Authenticator` properties.