Initialize ONE_METER_RSSI constant

[suidpit]

We need to have a constant, ONE_METER_RSSI, for our distance calculation measure. This constant could be hardcoded, but this could be problematic due to differencies between boards and the environment in which the boards are deployed. A more robust approach would be to have the server initialize those values for every board when the system first boots: for instance, one or two initial flushes could be used by the server (using some reference devices) to tune it.

[marcomicera]

For a specific (mobile) device, the core server should ask us to place that particular device at 1 meter from each board, at different steps. During this measurement time, the server should take not of the average 1-meter-distance-RSSI value.
core should finally have a data structure mapping boards' MAC addresses to their ONE_METER_RSSI values.

[marcomicera]

Waiting to be merged (#67).

[marcomicera]

-51 seems to do the job now. We have #67 ready to be merged in case we wanna include dynamic calibration into the system.