Audio input support

After reading through ssfrr/AudioIO.jl#1, I've done some work on getting audio input to synchronize between the PortAudio callback in C and Julia. Using this setup it is possible to get (mono) audio from the default input device, into the global mixer in Julia where it can be processed by AudioNodes and returned back to the C callback.

The method uses a single buffer to share both the input and output data between Julia and the C callback. This is probably not optimal with respect to latency, but I feel it results in a quite elegant solution. I haven't done any tests with respect to latency yet, but I'd say that this method can introduce a single buffer duration of latency at most.

I realize there are some cosmetic changes to some of the files as well. I've tried to stay close to the original conventions. If you'd like me to change anything, just let me know. If anything can be done in a better way I would like to know as well, since this is my first Julia code :smile:.

I have included an example script with a mutable audio node that shows this works. I have tested this on a 64 bit machine running Ubuntu 13.10.