Performance and Header Improvements

[petertait]

This is a big chunk of changes so would be good to get a second pair of eyes on this. Changes look at making the site a bit more performant and to tweak the header.

Performance changes

• Critical css is inlined within the – this is to be nice to Google Pagespeed and to speed up initial page load by not blocking initial page rendering.
• CSS is loaded asynchronously using html5 preload attribute – if browser doesn't support this cssload provided fallback. If JS has been turned off then the css will be loaded like normal. This change is for same reason as above.
• Typekit is loaded asynchronously – same as above.
• Removed recruitee's external styles. Instead styles were cherrypicked and placed into a scss file.
• Client logos have been put into a sprite.
• Images have been further optimised.
• Added lazyloading to the Careers page to reduce initial load from 5mb to 500kb or so... winning.

  Header changes

• JS for this has been refactored a little to fix duplication on services page.
• Header now changes colour on all pages – this also fixes a js error that was reported.
• Header no longer auto-hides, this makes the links always accessible and simpler to use.

Other changes include slight visual tweaks and minor refactoring.

[export-mike]

@petertait just seen the notification for this pr and spotted the 'Critical Css' Buzzword just reminded me of this https://github.com/addyosmani/critical might be useful might not be ¯(ツ)/¯