BG data plotted on a log scale

@HowardLook please sign off on the list of implemented changes here (to close #44) and also whether this change is considered to solve issue #34 with respect to the typography on the y-axis (referencing its appearance [here](http://tidepool-org.github.io/tideline/example/bg-log-scale-2.html) which is the dimensions that blip will be updated to soon)

What's implemented here is:
- BG data plotted on a log scale, with the min and max of the y-axis set to the extent of the data that's loaded (i.e., accessible from the scrollbar)
- average BG "floater" also on a log scale