TinkerForge binding enhancement: reconnect support for IO16

New optional configuration variable "keepOnReconnect" for type io_actuator.
New value "keep" for the "defaultState" variable of io_actuator.
- keepOnReconnect=true: keep the current state of the actuator after a reconnect. The initial value when openHAB starts is the defaultState.
- defaultState=keep: the current state of the actuator won't be changed even at openHAB startup.

Example:
tinkerforge:io16outa2.uid=efY
tinkerforge:io16outa2.subid=outa2
tinkerforge:io16outa2.type=io_actuator
tinkerforge:io16outa2.defaultState=keep

tinkerforge:io16outa3.uid=efY
tinkerforge:io16outa3.subid=outa3
tinkerforge:io16outa3.type=io_actuator
tinkerforge:io16outa3.defaultState=true
tinkerforge:io16outa3.keepOnReconnect=true