oauth support

The main purpose of this fork right now is to add oauth support before Twitter turns off basic authentication api access on June 30, 2010.

I will support oauth by way of the oauth gem and assume that all key management (including generating request and access tokens) takes place directly with oauth. The end result of the authentication process is an OAuth::AccessToken instance which exposes get/post/etc methods wrapping the corresponding Net::HTTP objects and methods. This will require changing the Twitter::Client#create_http*request methods and perhaps also the pattern of passing a block to Client#http_connect but shouldn’t require any changes to the public interfaces beyond a new parameter to the constructor passing in the OAuth::AccessToken object.

Connection management (setting proxy, etc.) can probably stay inside the Twitter gem although this can also be performed on the AccessToken object directly.