post https://external-api.xtremepush.com/api/external/analytics/unique_sessions
Returns info on unique sessions over a given time period.
The aggregation and grouping of the returned values is controlled by the optional time period given in
params:
- one day will return values per hour
- up to 30 days returns values per day
- greater than 30 days returns values per month
- greater than a year returns values per year