Adds in support for getting the system id of the BLE device.

It doesn't appear that the serial number field is unique for the TailorToys BLE device. 

From the 

> **System ID** This field is a 64 bit number which uniquely identifies a particular device. It is built from a combination of the internal MAC address and other values. This value should be used as an identifier only, the actual value has no pattern to it and is not a “serial” number.

The information regarding this value, 0x2A23, can be found here: 
https://developer.bluetooth.org/gatt/characteristics/Pages/CharacteristicViewer.aspx?u=org.bluetooth.characteristic.system_id.xml

I added in another method to BLEService, get64bitValueForCharacteristic, to get the 64bit value into a long.