Two new funcs for HTTP GETting URLs.

1) simpleDownloader downloads one URL on each invocation. It is a simple
wrapper aroung http.Get

2) parallelDownloader is meant to download many URLs simultaneously. It
isn't inherently parallel, but many invocations of it can be run as
goroutines because all data is exchanged through channels. It should
really be called canBeParallelDownloader, but that's an ugly name.
Download funcs are in a separate file now.