Create a new core-layout layout

Create a new layout which provides the toolkit grid mixins as classes.
It also ensures the wrapper is the correct width. This can then be used
by new applications which want to use the new grid styles without having
to redefine the grid classes before the start.
Add some basic docs around the slimmer templates

So that people know what the templates are for and when you might use
them introduce some basic documentation describing when you might want
to use them.
Add basic reset to remove spacing and font-sizes

Remove extra padding and margin by default from all elements. This will
then let us explicitly add the whitespace that we want for each element.
Make strong bold
Merge pull request #483 from alphagov/column-layout

Create a new core-layout layout