Change the user agent to be more like a browser's user agent

Since setting up gAnalytics for the plugin update system, most of the browser information is useless, since gAnalytics doesn't understand the current format we use for the User-Agent:

`Quicksilver/4008 OSX/10.9.0 (x86)`

So I've updated the user agent to more closely mimic that of typical browsers [like Safari](http://www.useragentstring.com/pages/Safari/) meaning gAnalytics should better understand it:

`Quicksilver/4008 (Macintosh; Intel Mac OS X 10_9_0; cy-gb) (like Safari)`

Things to note:

* Safari uses `like Gecko`, I've put in `like Safari` - this should help with any browsers that reject requests based on User Agent. It's pretty [interesting why Apple did it in the first place](http://www.nczonline.net/blog/2010/01/12/history-of-the-user-agent-string/) (search for Webkit). I think it's fine for us to pretend to be Safari? :P This is mostly useful when using QS to browse sites I guess
* The locale in the user agent doesn't necessarily show the preferred locale of the current user, but shows the language they are using Quicksilver in. This was intentional. I think it's more interesting for us to know that "90% of people in Russia are using Quicksilver in Welsh" or whatever as opposed to "99% of people in Russia have their OS language set to Russian".
* The word `Intel` is hard-coded, no need to test for PPC or x86_64 now (we can tell from the QS version)