added highlight to element on hover by decreasing the opacity of the bac...

...kground elements using a global higlight function

@jebeck can you look at it, i think its ready but i'd like to hear what you think of the code and the resulting interaction.
Oh, if we keep this way of generating the common class across bolus and carbs events, can we factor out a function for generating it? In the future maybe we'll include the joinKey in the data for bolus and carbs and so we could use that instead of the time, and it'd be best to only have to change it in one place ;)