Timeout support for wait_for_edge (replaces PR #62)

[jpbarraca]

This change makes it able to wait for an edge change but bail out if no change is detected after a timeout.
The code should be mostly backward compatible but enables programs to recover when the hardware goes weird.

The function wait_for_edge will now admit a timeout value and will return 0 or -1 . If the return is 0, an edge change is detected. If the return is -1 a timeout occurred.

Previous code should work without changes, waiting for a change forever.
Same as PR#64 but with a shorter change history.

[pdp7]

@jpbarraca thanks, I'll test it out tonight

[pdp7]

@jpbarraca Thank you for this pull request.

Tests have passed OK:

root@beaglebone:~/jpbarraca/adafruit-beaglebone-io-python/test# py.test
============================================================================ test session starts ============================================================================
platform linux2 -- Python 2.7.9, pytest-3.0.4, py-1.4.31, pluggy-0.4.0
rootdir: /root/jpbarraca/adafruit-beaglebone-io-python, inifile: 
collected 57 items 

test_adc.py ......
test_gpio_input.py ..
test_gpio_output.py ......
test_gpio_setup.py ..........
test_led.py ..
test_pwm_setup.py ..........................
test_spi.py ..
test_uart.py ...

========================================================================= 57 passed in 4.76 seconds =========================================================================

This test program runs OK:

import Adafruit_BBIO.GPIO as GPIO
GPIO.setup("P9_42",GPIO.IN)
GPIO.wait_for_edge("P9_42", GPIO.FALLING, 2000)

root@beaglebone:~# python ./wait-for-edge-timeout.py
root@beaglebone:~# uname -a
Linux beaglebone 4.4.30-ti-r65 #1 SMP Tue Nov 8 17:28:39 UTC 2016 armv7l GNU/Linux
root@beaglebone:~# cat /etc/dogtag 
BeagleBoard.org Debian Image 2016-11-21
root@beaglebone:~# cat /etc/debian_version 
8.6

[jpbarraca]

Great!