Add a AuthenticatedRoundRobinProxyHTTP method to authenticate proxies

[wowinter13]

This PR addresses the issue #343

Hey!
Right now colly doesn't have an ability to authenticate proxies out of the box. So I added this functionality:)
Also I decided not to modify proxy_switcher.go because it would break an example without having correct credentials.
(Also my deepest gratitude to @andystroz)

[wowinter13]

@asciimoo Also we could rewrite RoundRobinProxySwitcher and make credentials to be nullable. Therefore we would be able to delete AuthenticatedRoundRobinProxySwitcher. (but imho for now that is better to have both methods)

[WGH-]

I personally don't see why it has to be built in into Colly. You already can install a custom HTTP RoundTripper with any behaviour you want.

[wowinter13]

Hey @WGH- !
Most crawlers have the functionality to authenticate proxies. At least, Scrapy.
The situation is a little bit inconvenient when you have a built-in method to rotate proxies and don't have any methods to do basic auth for these proxies.

Also if you check the issue #343, you will see some demand for this feature.

[wowinter13]

@asciimoo ?

[samirsss]

Any thoughts on this approach - i'd like this to be included as part of Gocolly too.