Added conversion of timestamp for pre-Jellybean MR1 APIs addressing issue #31, now on the correct branch
Added required permissions to start a Wifi scan and to read scan results for issue #16
Added getter for Wifi Service
Included WifiHolder in sensor thread setup
Added method to serialize a list of scan results at a specific time for issue #16
Implementation of a Wifi Holder that periodically checks the list of available Wifi Access Points
Added constant for Wifi Holder delay
Some Corrections
* Refacoring. 
* Comuting the correnction of the timestamp once at construction time.
* Catch unrgisterReceiver error.
Merge branch 'feature/newSensors' into dev