Menu button functionality:
- Added script in header.html to listen for events and handle them
- Added styles for the menu when in its new postion
- Changed some styles for nav--hidden so it animates nicely in new position
- Added styles for the mask
- Added a new close button in the nav menu
- Added styles for the close button
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- Still work in progress, needs checking at this stage
Moved scripts to _scripts/

Tweaked JS for tweaks to menu
Removed close button
Tweaked  stylings of overlay

Renamed it from #mask to .shadow-overlay
Tweaked stylings for slide out menu

Cleaner look with white
Resized for different screen sizes
Added transitions and css transform
Merge pull request #7 from TEDxUniversityofPortsmouth/menu

Menu button functionality (for issue #2)