Merge pull request #21 from Jemgoss/master

Fix mapping of GPIO header pins 24 and 26
eventfd.eventfd renamed to eventfd.Semaphore, ack renamed to receive(), non-blocking implemented
Merge branch 'master' of github.com:quick2wire/quick2wire-python-api
a simple selector that makes epoll more convenient
fixing gpio tests
fixing GPIO tests
Button blink example uses the new Selector class instead of epoll
Selector has timeout and can remove event sources
Renamed loopback test to follow conventions
error checking on creation of eventfd file descriptor
can import eventfd.Semaphore directly from selector module.  The plan is that quick2wire.selector will provide the Selector class and also some useful classes to use with a Selector, such as Semaphore, Timer, etc.
Ctypes binding to Linux timerfd
fixed ctypes library loading to set errno on failure
Timer object that works with the Selector class.
Pulled lookup of syscalls via ctypes into quick2wire.syscall module
Timer can be scheduled for one-shot or repeating alarms.  Can be given
offset and interval and then started/stopped.
Timers work with epoll and the Selector class.  Timer is exported from
the quick2wire.selector module.
Example of using selector to handle timer and GPIO interrupts
Selector.add(...) defaults event mask to INPUT|ERROR
Documentation for Semaphore, Timer, Selector.
Renamed Semaphore.receive() to wait()
Can create Semaphore with initial value
fixed some formatting in the docstrings