Pylibftdi

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* added some unit tests
* API changes:
* when opening a device with a device_id parameter, this will now check
against both serial number and (if that fails), the device description.
Opening by device type (under the same proviso that an arbitrary device
will be selected if multiple matching devices are attached as when no
device_id is given) is frequently easier than matching by serial number.
* added flush(), flush_input() and flush_output() operations. This is
modelled after the pySerial API providing separate flush operations, and
gets the Device API closer to that of files.
* increased file-API compatibility, with line-oriented methods and iteration

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* support multiple attached devices
* API changes:
* separation between driver and device. Generally, now use Device() /
BitBangDevice() rather than Driver / BitBangDriver(), though older code
_should_ still work via some shims (though with Deprecation warnings).
* first parameter in Device() and BitBangDevice() is a device ID - the serial
number string of the target device.
* open() is generally no longer required on Devices. This is to more closely
model the file() API.

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* same source now works on both Python2.6+ and Python3.

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* fix various bugs found by pyflakes
* improve module organisation, while providing interface compatibility

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* fix release issue

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* fixed embarrassing bug which caused things not to work on Linux
(is now find_library('ftdi') instead of find_library('libftdi'))
* lots of error checking, new FtdiError exception. Before it just
tended to segfault if things weren't just-so.
* get_error() is now get_error_string(). It's still early enough
to change the API, and if I thought it was get_error_string
multiple times when I wrote the error checking code, it probably
should be the more natural thing.