Add datatype.h to share the definition of SENSOR_DATA.
Add debug.h to share the definition of debug print macro.[D
To use debug.h and datatype.h on gardeinig node.
To use debug.h and datatype.h on powerplant node. issue #20.
To use LED pin definition.
Add XBee initialization.
To see main loop interval setting.
Modify some debug messages.
Implement to read current/voltage sensor values.
Made IElectricPower class as abstract class.
Add multiple value as sensor data structure.
Merge branch 'make_powerplant_node2'

Conflicts:
	nodes/gardening/gardening.h
	nodes/gardening/gardening.ino
	nodes/libraries/debug/debug.h
	nodes/powerplant/powerplant.h
	nodes/powerplant/powerplant.ino