Read from position 0 of value file.

Otherwise this results in a EOF error on all but the first read.
Compare gpio value with '0' as opposed to 0

Reading the value file returns a 1 char string, not an integer.
Pause after exposing pin.

The gpio driver un Raspbian creates the gpio subdir with root-only
permissions, and then shortly afterwards updates these to allow
access to the gpio group.

Without this delay, opening a pin fails with a permission denied error
unless run as root.
Merge remote-tracking branch 'origin/gpio_reading_fixes' into working_master
Merge branch 'pause_after_pin_expose' into working_master
Add mock_gpio using gomock