Added command-line parameters

For output filters, continuous output, choose device file.
README updated: explain command line switches
Rasperry pi rev 2's I2C port is bcm2708_i2c.1
Pull the reading logic out of main()
Handle I2C bus names containing spaces
With both -T and -H options, show temp and humidity
Specify the device name with the -d option

To make the specification of a device or bus name more uniform.
Specify the I2C slave address with the -a option
Update the README and munin_plugin.sh for option changes