Parameterized ntpdate path
Removed conflicting SPI device ids in overlay.  Made overlay loading optional when creating a SPI device.
Example:
a = SPI.SPI(0, 0, False)  # This will NOT automatically load the device tree overlay for spi0
Made 'build' the default task in the Makefile
Merge branch 'master' of git://github.com/adafruit/adafruit-beaglebone-io-python