Make document footer component handle rtl content

For RTL content change the text direction and swap the floats so the
reading edge is the other side of the page. There will be a future bit
of work for adding translations to the strings in the component.

This is dependant on [this enhancement][1] making it into the 
govuk_frontend_toolkit. It will need to be amend to use that once it is
available.

[1]: https://github.com/alphagov/govuk_frontend_toolkit/pull/151
Do we care about [odd IE6 behaviour](http://www.paulirish.com/2008/the-two-css-selector-bugs-in-ie6/) here for multiple class selectors?