Simplify markup and styling of admin header

* The primary navigation of Whitehall is nothing like a Bootstrap navbar. If we don’t have to override all those Bootstrap styles the CSS can be simplified a lot.
* This also fixes the rendering in IE7 when all tabs are visible
* No longer hides tabs at smaller page widths
* Keeps the header looking as it did before, minus the horrible text shadows and pipes (|) that would underline on hover
* Remove or clarify CSS that was also in `layout.scss`

Implementation:
* Use a new `masthead` class, and split into two sections: details and tabs.
* Simplify markup by removing nested containers
* Make visuallyhidden a re-usable class by taking it out of the nav specific styles

Before:
![screen shot 2014-07-18 at 18 21 23](https://cloud.githubusercontent.com/assets/319055/3656359/7858b2f0-1185-11e4-844f-4be4df443897.png)

After:
![screen shot 2014-07-18 at 18 21 09](https://cloud.githubusercontent.com/assets/319055/3656358/78589928-1185-11e4-90d6-d014fed8a275.png)

Collapsed:
![screen shot 2014-07-21 at 15 30 57](https://cloud.githubusercontent.com/assets/319055/3656371/94dee782-1185-11e4-9bff-5fa1096ec3ca.png)
![screen shot 2014-07-21 at 15 31 08](https://cloud.githubusercontent.com/assets/319055/3656370/94de5cd6-1185-11e4-9135-10d495ff9516.png)

Incase you didn't know we have a mixin to help with inline-block for IE: https://github.com/alphagov/govuk_frontend_toolkit/blob/master/stylesheets/_shims.scss#L6-L35