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 and I looked at this, and he agrees that we should:
- take out highlight for CGM data
- use the default 60% for SMBG
- use 40% for boluses and carbs