friend tagging feature

Okay, this is the PR that requires discussion.

I was annoyed that m.facebook.com doesn't support tagging friends in status updates and comments the same way you can on the desktop site (i.e. just by typing their name). I discovered that [it _is_ possible to do it on mobile](https://webapps.stackexchange.com/questions/27576/how-to-tag-friends-via-mobile-web-site-m-facebook-com). So I wanted to add this functionality to Tinfoil.

The problem is that it requires cooperation with a Facebook app, so that it can have access to your friends list. The app I created is [this one](https://developers.facebook.com/apps/266257470187937) and it has literally no code in it -- it exists to create an API key, and then it redirects to a URL that doesn't exist, and Tinfoil intercepts that.

So, see what you think of the tagging function here! You have to go to the preferences to turn it on, and then while you're typing a status update or comment you click on the "Tag Friend" menu item. When you select one to tag, it inserts a cryptic code into the current text field, and that turns into a tag once you submit it.