Added feature to allow config to specify namespace
Fixed Namespace stat reporting, improved delta calculations.

Note: Nagios configs need to be regenerated for namespace stats to work
      There were issues in both asnagios.py and check_aerospike.py
      related to this.

Deltas are now calculated over many time slices. To compute a delta we
first determine the min and max value of the time slices and then
calculate the change between the youngest(min, max) - oldest(min, max).
Updated documentation to reflect known issue resolutions.