Pifacecommon

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- Added daemon flag for PortEventListener.

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- Fixed bug with new Device Tree (Pi2) by changing GPIO_INTERRUPT_DEVICE
from `/sys/devices/virtual/gpio/` to `/sys/class/gpio/` and changing udev
rule.

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- Support varying listeners.

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- Added deregister to interrupts.

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- Fixed SPI file descriptor bug when closing.
- Fixed issue 14.

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- Ignored "Interrupted system call" error in `watch_port_events`.
- Rewrite main functions into chip specific (MCP23S17) class.
- GPIOInterruptDevice class replacing core GPIO enable/disable functions.
- SPIDevice class replacing spisend function. Can now add in spi_callback
function which is called before each SPI write.
- Updated installation instructions.