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.
@brandonarbiter @HowardLook how do you feel about this, approximate time matching for generating "combo" tooltips for bolus and basals? Get rid of it, or keep it around because it may be useful one day for manual logging data sources?