Remove util.js, replace its methods with npm modules, inline code

* Replaced util.inherit method with <i>inherit</i> module calls
* Replaced util.on and util.load, which were used only in /lib/transports/flashsocket.js to make sure the swf file for flash support is downloaded once the document's fully loaded (added the support in the relevant place in flashsockets.js)
* Replaced util.ua with inline UA detection (used sparingly in one or two places, so makes sense to be inline)
* Removed util.js