Allow cookies to be sent in XMLHttpRequest handshake; see https://github...

....com/LearnBoost/socket.io-client/issues/344

With these changes, can now send a cookie with the XHR handshake, allowing for persisting a user session.

Note about dependency upgrade:
Had to update XMLHttpRequest module to 1.6.0, to be able to disable forbidden headers in an XHR request.