Add support for installing docsets into Xcode 5.

As you know, Xcode 5 won't have UI for adding docsets, so the instructions in docset page's Xcode button, to "Copy & Paste into Xcode's Prefs", won't work anymore. This pull request addresses that.

I wrote a small (and free, of course) application to deal with this, [Docs for Xcode](http://documancer.com/xcode/). It has a custom `docs-for-xcode://` protocol for adding feeds, so it's possible to make installation of docsets as seamless as with Dash. This patch adds a link to cocoadocs.org's Xcode popup very similar to the existing Dash support:

![xcode_popup](https://f.cloud.github.com/assets/145881/1132549/eb3c060a-1bcc-11e3-8a4a-c9894ed3d845.png)

(I thought it would be better to keep existing UI for Xcode 4, even thought the app works with it as well.)