Adding StackExchange

I have added support for StackExchange. Details on StackExchange OAuth2.0 authentication can be found here - http://api.stackexchange.com/docs/authentication. 

Note: 
1. StackExchange provides a "key" in addition to client_id and client_secret. This seems to be a mandatory parameter for the /me API(user info API) in addition to the access_token.

2. Empty scope is enough to get basic user information from the /me API. For other information scope can be set to read_inbox, no_expiry, write_access and private_info(see the authentication doc above).