ETENTION_HOURS is added to SensePrefs.
The duration of caching data locally for sensors can be set from applications by setting
desired hours in RETENTION_HOURS. Default is set to 24.
fixed indent
Merge branch 'f-cache-duration' into authentication_api_dynamic_cache
Merge pull request #112 from senseobservationsystems/authentication_api_dynamic_cache

Authentication api dynamic cache