Scale circle size by median per lender (per metro, if appropriate)

Instead of using a fixed scalar, determine the size of bubbles by the number of loans that lender makes (in the metro, if a metro is selected). Divide a large constant by the median number of loans per tract so that a medium-sized circle's size is relative to the lender. Adds db indexes to speed the query up.

Look at these two lenders' similarly sized bubbles (with very different absolute numbers).
![screen shot 2014-08-20 at 3 16 13 pm](https://cloud.githubusercontent.com/assets/326918/3988317/94d0b2da-28ae-11e4-8269-ffaa349d4096.png)
![screen shot 2014-08-20 at 3 16 37 pm](https://cloud.githubusercontent.com/assets/326918/3988320/9922cecc-28ae-11e4-8218-715c13a0330f.png)