cache board model during initialization

[pdp7]

While investigating #190, I realized it is very inefficient to check for board type inside gpio_set_value():
https://github.com/adafruit/adafruit-beaglebone-io-python/blob/master/source/event_gpio.c#L405

The function beaglebone_blue() executes popen() to run grep:
https://github.com/adafruit/adafruit-beaglebone-io-python/blob/master/source/common.c#L602

This should be cached so that it only needs to be done once when Adafruit_BBIO is initialized.

[pdp7]

Timing test shows large performance improvement after commit 3281479

debian@beaglebone:~/ssh/adafruit-beaglebone-io-python$ python3 ~/time-test.py 
time to complete 100 sets =  0.006628266011830419

debian@beaglebone:~/ssh/adafruit-beaglebone-io-python$ python ~/time-test.py 
('time to complete 100 sets = ', 0.0067021846771240234)

Results before commit 3281479 with performance regression:

debian@beaglebone:~/ssh/adafruit-beaglebone-io-python$ python ~/time-test.py 
('time to complete 100 sets = ', 1.5183091163635254)

debian@beaglebone:~/ssh/adafruit-beaglebone-io-python$ python3 ~/time-test.py 
time to complete 100 sets =  1.3909540939785074

time-test.py

import Adafruit_BBIO.GPIO as GPIO
import timeit

pin = 'P9_14'

GPIO.setup(pin, GPIO.OUT)

def setoutput():
  for x in range(1, 10):
    GPIO.output(pin, True)
    GPIO.output(pin, False)
    #run 5 times, 10 x 2 x 5 = 100 outputs

result = timeit.Timer(setoutput).timeit(5)
print('time to complete 100 sets = ', result)
GPIO.cleanup()

/opt/scripts/tools/version.sh

git:/opt/scripts/:[6c2688b0be448b7bb9ca18849b430d496a84acb4]
eeprom:[A335BNLT000C3014BBBK1316]
model:[TI_AM335x_BeagleBone_Black]
dogtag:[BeagleBoard.org Debian Image 2017-10-17]
bootloader:[microSD-(push-button)]:[/dev/mmcblk0]:[U-Boot 2017.09-00002-g0f3f1c7907]
bootloader:[eMMC-(default)]:[/dev/mmcblk1]:[U-Boot 2017.09-rc2-00002-g84a7f2]
kernel:[4.14.0-rc7-ti-r6]
nodejs:[v6.11.5]
uboot_overlay_options:[enable_uboot_overlays=1]
uboot_overlay_options:[uboot_overlay_pru=/lib/firmware/AM335X-PRU-RPROC-4-4-TI-00A0.dtbo]
uboot_overlay_options:[enable_uboot_cape_universal=1]
pkg:[bb-cape-overlays]:[4.4.20171027.1-0rcnee1~stretch+20171027]
pkg:[bb-wl18xx-firmware]:[1.20170829-0rcnee1~stretch+20170829]
pkg:[firmware-ti-connectivity]:[20170823-1rcnee0~stretch+20170830]