feat(toolbar): add button to hide/show toolbar

**menu.jade**
* Add toggle button to toolbar
* Add container to hide the rest of toolbar without hiding the toggle
button

**menu.styl**
* Add toggle button styles
* Update toolbar styles to account for toggle button

**header.styl**
* Update rules for adding and removing padding from .header-wrap when
toolbar is toggled.

As requested by @deilann, I’ve brought back a button to toggle the visibility of the toolbar. Haven’t activated it for mobile yet as I can’t think of a way to stop the button obscuring the header when the toolbar is hidden.

Once I’ve got the mobile toolbar on one line, I might revisit it as it may not be necessary then anyway. Open to suggestions though.

@lefnire @paglias I’ve used a variable to hide/unhide the toolbar. Is that OK or should I be doing it via a function instead?

**Toggle button – default**
![toggle-inactive](https://cloud.githubusercontent.com/assets/3317594/2866611/e4eec9b2-d22a-11e3-90e4-02270a4d6736.png)

**Toggle button  – on hover**

![toggle-hover](https://cloud.githubusercontent.com/assets/3317594/2866612/e4eee852-d22a-11e3-8fab-57637cf1dae3.png)

**Toggle button  – activated**

![toggle-active](https://cloud.githubusercontent.com/assets/3317594/2866610/e4e92e3a-d22a-11e3-95f6-72cdb3f07a11.png)

**Toggle button  – activated, fixed to top right corner when scrolling down page**

![toggle-scrolled](https://cloud.githubusercontent.com/assets/3317594/2866613/e4ef182c-d22a-11e3-8891-0e6bcef89fc7.png)