- Add option to control which percentiles for distribution metrics are calculated on the the client and to be emitted as a gauge

Fixes DataDog#17

test name from percentile)

- Change how percentiles are configured
- Change metric suffix

- Configure using strings to remain consistent with other datadog
config apis

- Change standard deviation to 1 and change comparison of expected and got to be within
a given threshold
- Link to the cumulative standard table to verify tests