doc: pwm constants
doc: group the bbb constants together
bbb: disable the pwm pin when closed
move the Register calls to the bottom
bbb: add a comment explaining why we cannot disable the analog and pwm
modules by removing them from the slots file
remove the old spi driver
- use glog instead of Debug flag
- use glog instead of log
- make code conform to the Go code guidelines
unified servo and analog output support for pwms
gpio: fix a bug which prevented digital pins reads from working correctly

when reading from the value file, it is important to seek back to the front
so that we get the fresh values from the kernel driver for every new read
gpio: remove edge file handling from digital pin

to be handled in future
samples: enable flag parsing

this will allow people to use glog options (like -v=3) to increase
verbosity of the log output
bbb: initialise at the start of SetMicroseconds

this will setup p.period to the default if not already done
us020: make the state comparisons clearer
gpio: provide an api to allow performance timing of pulses

the us020 driver is the first consumer of this functionality
gpio: cleanup test code
ci: fix travis.yml so that failure is captured properly

subtle errors in the config were causing test failures to be ignored
gpio: fix failing tests
samples: fix servobbb sample
samples: servobbb cleanup
gpio: create types for the pin factory methods