fix(connection): set token expiration date for GCE

A response from the GCE metadata server responds with data in a
different format from that of the GAPIToken() request. Instead of
looking for a property that doesn't exist `token_expires`, we now
calculate the expiration timestamp from `expires_in`.

For more, see [Authenticating from Google Compute Engine][auth].

[auth]:https://developers.google.com/compute/docs/authentication#applications

Fixes googleapis#212

lib/common/connection.js

@@ -165,7 +165,7 @@ Connection.prototype.fetchToken = function(callback) {
         callback(err);
         return;
       }
-      var exp = new Date(body.token_expires * 1000);
+      var exp = new Date(Date.now() + body.expires_in * 1000);
       callback(null, new Token(body.access_token, exp));
     });
     return;