Adding ability to specify an agent for OAuth2 requests

lib/oauth2.js

@@ -15,7 +15,16 @@ exports.OAuth2= function(clientId, clientSecret, baseSite, authorizePath, access
   this._authMethod= "Bearer";
   this._customHeaders = customHeaders || {};
   this._useAuthorizationHeaderForGET= false;
-}
+
+  //our agent
+  this._agent = undefined;
+};
+
+// Allows you to set an agent to use instead of the default HTTP or
+// HTTPS agents. Useful when dealing with your own certificates.
+exports.OAuth2.prototype.setAgent = function(agent) {
+  this._agent = agent;
+};
 
 // This 'hack' method is required for sites that don't use
 // 'access_token' as the name of the access token (for requests).
@@ -129,6 +138,11 @@ exports.OAuth2.prototype._executeRequest= function( http_library, options, post_
 
   var result= "";
 
+  //set the agent on the request options
+  if (this._agent) {
+    options.agent = this._agent;
+  }
+
   var request = http_library.request(options);
   request.on('response', function (response) {
     response.on("data", function (chunk) {