Add fields to the API for the Poll object

This commit adds two fields to the /api/v1/polls output. These fields
are the response_count, and the response_percent which are exactly that.
The two fields can be used by a consumer of the API to get simple
overall statistics on the performance of a poll.
Merge branch 'master' of https://github.com/unicefuganda/ureport
Add additional fields to the Polls API result