Adding good support for css noise

I've added a highlight group to ; and : within css declarations
Adding / to cssNoise

The / is used in places for various screen aspect ratios and also font
declarations to separate font size and line height.
Fix for cssDefinition

By adding keepend to cssDefinition we ensure that if the final attribute
in a css definition does not include a ;, it will still be closed by the
cssDefinition's final }.

As per the css spec, the final property does not require a ;, and this
is still technically valid css.
Merge pull request #21 from amadeus/css-noise

Adding good support for css noise