v2.6 Keepalive feature added

A new feature has been added. To use this feature you must use either FirmataPlus or FirmataPlusRB. The keep alive feature sends a periodic keepalive message to the arduino (the period length can be from 1 to 10 seconds and is configurable as a parameter in the new keep_alive method. The arduino expects to see the message within the period time frame, and if it does not, it resets itself.

Once keep_alive has been actuated, it can be turned off by specifying a period of zerio.