Add sysfs_adc_getvalue to sysfs.c

sysfs_adc_getvalue reads the adc values from the respective sysfs
entries for the bacon cape.
Add member "analogChannel" to pin descriptions structure

analogChannel is passed as an argument to sysfs_adc_getvalue.
Add file wiring_analog.c (and .h) and function analogRead

analogRead calls sysfs_adc_getvalue and scales down the adc values
to 10 bits
Remove analog functions from wiring.h

This has been deprecated and the prototypes should now be in
wiring_analog.h
Add analog example to BaconDemo

The slider value is read and displayed on the console