Add inner and outer block mixins to align content

- Add a new mixin, outer block - to set a max width of 1020px and a
fixed with for IE8 and below
- Add a new mixin, inner-block, to set left and right padding to match
the header and footer
- Change phase banner to use inner-block mixin

Once these mixins are merged into the front end toolkit, they can then
be used the to align the cookie banner and footer in the govuk_template.

A test page showing this in action exists here:
http://govuk-elements.herokuapp.com/examples/template/
Update README with layout explanation and example
Add civil service brand colour
Merge pull request #101 from alphagov/civil-service

Add civil service brand colour
Add alpha/beta examples to read me
Merge pull request #100 from gemmaleigh/align-all-the-things

Add inner and outer block mixins to align content
Include conditionals in grid_layout

For @include ie-lte(8).
Fix syntax of extending the contain-floats shim

Also include the necessary file. Fixes #103.
Merge pull request #104 from alphagov/fix-grid-layout-extend

Fix _grid_layout @extend