Salesforce OAuth2 support

Hi, I prepared and tested a configuration for Salesforce's OAuth2 main and sandbox backend.

I based the settings on the following Salesforce documentation and my own testing:

* [Understanding the Web Server OAuth Authentication Flow](https://www.salesforce.com/us/developer/docs/api_rest/Content/intro_understanding_web_server_oauth_flow.htm)
* [Understanding the OAuth Refresh Token Process](https://www.salesforce.com/us/developer/docs/api_rest/Content/intro_understanding_refresh_token_oauth.htm)

Let me know if there is anything I missed or feel free to pull it in.