Off screen nav restricted

Off screen nav is now toggled by the show/hide menu buttons only and
clicking "anywhere on the screen" has no effect.
Style guide updates

Made style guide independent and added more style guide styles. Also
fixed font issue with default variable settings.
Mixin - Margin update

Updated margin to allow auto
Mixin - Padding update

Updated padding mixin to allow for auto
Mixin - Box shadow update

Updated box shadow to take a single string rather than set values.
CSS tidy up

General updates of CSS removing no longer needed values.
Updated style guide tiles

In progress of adding new style guide tiles for quick view of core
variables.
Updated default colours in styleguide

Updated default colours and created temporary classes for style guide
tiles.