Percentage columns

changes to https://github.com/dimagi/commcare-hq/pull/5132

include option to show numbers in a column as percent of total in its own field, without affecting aggregation

very simple example:
![screen shot 2014-12-19 at 3 37 32 pm](https://cloud.githubusercontent.com/assets/1757035/5510863/009fbf20-8795-11e4-9b12-b0ab4dbf92f8.png)

same example without percentages:
![screen shot 2014-12-19 at 3 38 51 pm](https://cloud.githubusercontent.com/assets/1757035/5510872/27ed1abe-8795-11e4-8dc5-4e8e4deb47c2.png)