Bugfix fork addressing urllib exceptions

[fish2000]

I was having trouble running yolk -U, as the entire process would halt whenever the occasion of a transient urllib2.URLError would bubble up to the CLI:

… so I forked yolk3k and vendored in retrying – which I used here and here, rather coarsely, to elide halting the program on these exceptions.

The fork works as it should, but it seemed like a biggish change for an unsolicited PR; if you are interested in integrating this stuff upstream, I’d be happy to clean it up and squash it down. Indeed. Thanks in any case, yes!