[Docs] Add language switcher to the navbar

@JohnONolan (as suggested on https://github.com/TryGhost/Ghost/pull/1649) I took a stab at tweaking the language switcher (inspired by Twitter). I think merely moving it to the top makes it useful/stand out enough. I decided not to use flags because [flags don't represent languages](http://flagsarenotlanguages.com/blog/why-flags-do-not-represent-language/). ISO country code and language code are different (e.g. Japan is JP but Japanese is JA), and a lot of the flag icons I found online use country code and not language code, so using flag icons might be too much of a hassle.

* Moved the language switcher to a partial and added it to the right hand side of the navbar
* **Translations** menu is now under the dropdown, to make a room on the navbar. It also makes sense because people who want to become a translator would want to see which languages are currently available.
* Shrink paddings on navbar items so they fit in one row on tablet
* I kept the dropdown on the bottom, there's no reason to remove it

### Collapsed

![image](https://f.cloud.github.com/assets/992008/1751106/ec44b950-65ca-11e3-9959-337753ac80d9.png)

### Open

![image](https://f.cloud.github.com/assets/992008/1751110/1e690300-65cb-11e3-980d-0855ef7ae0ae.png)

### In another language

![image](https://f.cloud.github.com/assets/992008/1751113/339de68c-65cb-11e3-8987-71930b9e3932.png)

### Mobile

![image](https://f.cloud.github.com/assets/992008/1751116/47cbab26-65cb-11e3-81f4-bcf739f7c914.png)