Deny upgrade when proxy doesn't support websocket

Some proxies - such as nginx and squid - don't support websocket. The client attempting to upgrade its transport caused the engine.io connection to break (see GH-177). THe server now checks within handleRequest to see if the transport has changed without an upgrade request - if this is the case, the upgrade request is denied (defaults to polling) and the connection is maintained
Merge branch 'master' of https://github.com/LearnBoost/engine.io into upgrade
Merge pull request #222 from nlagrow/upgrade

Deny upgrade when proxy doesn't support websocket