Plot background should be transparent

The example plots have transparent backgrounds if you look closely, so we
modify the scripts to produce the same. Comments have been expanded slightly
and typos corrected. Also updating the PNG output
Code & plot for time series of Global Active Power

Create a time series plot of Global Active Power, the household
minute-averaged active power in kilowatts.
Code & plot for time series of Sub metering 1-3

Create time series plots of Sub metering 1, Sub metering 2, and
Sub metering 3, the energy usage of the kitchen, laundry room,
and heater + A/C, minute-averaged in kilowatts.
Code and plot for 2x2 multiplot

Create a 2x2 multiplot comparing time series of various measurements.
Plot 1: Global Active Power, the minute-averaged household active power
	use in kilowatts
Plot 2: Voltage, the minute-averaged household voltage in volts
Plot 3: Sub metering, the energy submetering of the kitchen, laundry room,
	and water-heater + A/C
Plot 4: Global Reactive Power, the minute-averaged household reactive
	power use in kilowatts