Curl user agent

Hey @mitchellh,

As I pointed out to you on twitter once, cURL requests are not treated as "clicks" on URL shorteners like bit.ly. This patch changes `Vagrant::Util::Downloader` objects to provide an `--user-agent` parameter so that requests are properly tracked and I hope this doesn't bring in any side effect

I think this is really useful for box maintainers to find out if they can "deprecate" some previous box version. It seems to be really common on Vagrant community to serve base boxes from Dropbox and it doesn't support tracking downloads AFAIK and a URL shortener service can be a dead easy way of tracking downloads :)

PS: I didn't update the changelog since I'm not sure if this will make into 1.2.X series or if we'll wait until 1.3 ;)