Changed vars to one variable per var statement.

Because that's how Node.js does it and it's probably best to more or
less use the same code style.
Moved fileNameRegEx init to Downloader constructor.

There's no reason to keep rebuilding this on every next event.
Moved Downloader http.get() options to a variable.
Changed http.get() to use hostname instead of host.

The Node.js HTTP library documentation recommends using hostname over
host to be consistent with url.parse().
Refactored file exists check to separate if block.
Refactored Downloader switch logic into functions.