Pifacecommon

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- Added IODIR_FALLING_EDGE and IODIR_RISING_EDGE to replace IODIR_ON and
IODIR_OFF respectively. IODIR_ON and IODIR_OFF can still be used in the same
way as before. Falling/Rising Edge are for physical level 1/0, On/Off are for
logical (programmer) level 1/0.

Physical Level (pifacecommon.read_bit):
IODIR_FALLING_EDGE: 1 -> 0
IODIR_RISING_EDGE: 0 -> 1
Logical Level (pifacedigitalio.PiFaceDigital().input_pins[0].value):
IODIR_ON: 0 -> 1
IODIR_OFF: 1 -> 0

Remember that PiFace Digital Inputs are active low:
>>> pifacecommon.read_bit(0, INPUT_PORT)
>>> 1 physical
>>> pifacedigitalio.PiFaceDigital().input_pins[0].value
>>> 0 logical

- Fixed Debian package bug where setup script would not be executed.

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- Added debouncing with adjustable `settle time`.

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- Added timeout class (fixing Issue 2) in interrupts.
- Added support for interrupts on multiple boards.
- Interrupts must be enabled/disabled manually. Automatic handling of this
broke interrupts from multiple boards.

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- Moved version number to pifacecommon/version.py so that it can be read from
setup.py and bin/uninstall.py.
- Updated SPI help link to point to the new docs.
- Moved installation scripts into single file for Debian packaging.

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- Added version number in source.
- Added uninstall script.

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- Improved interrupts (different API, check the docs).
- Reduced scope of global variables from package to individual modules.
(Hiding namespaces from the end user is an attempt to simplify the interface
for children. However this package is not intended for that audience
and so messing with the namespaces only confuses things.)