Add header container

As discussed in [this comment thread](http://whoaverse.com/v/announcements/comments/28936/26728), I've added a header container that will align the logo, tab menu, and account information/mini login area to the rest of the content, as seen in this demo screenshot:

![](http://i.imgur.com/LRXOXAz.png)

I've written it in a way that they will be perfectly aligned while still allowing users to add custom-sized banners the same way as before.