fix cal/rawBg bugs, and scale by sgv/filtered ratio when available

This fixes the bug that was making the rawBG white dots continually shift.  Also, I took advantage of the fact that we have both filtered and sgv to allow us to determine the ratio that the Dexcom is currently using to scale sgv, and scale unfiltered by the same amount.  This makes the white rawBG dots disappear under the sgv dots when the Dex is using unfiltered to generate SGV.

![screenshot 2014-11-07 12 57 44](https://cloud.githubusercontent.com/assets/6982790/4957758/98ea81a2-66a7-11e4-8146-9c2056c2431e.png)