ofxAudioUnitFilePlayer pause / resume, priming, seeking

Hey all, this PR covers a few bugs and feature requests :)

@sebleedelisle There's a simple `pause()` function on ofxAudioUnitFilePlayer now which takes care of all that fiddly timey wimey stuff. It works like most media players, so calling `play()` then `pause()` then `play()` again will resume where playback stopped at `pause()`. Calling `stop()` will reset the playhead back to the beginning.

@jasonlevine I've added another function called `playAtSampleTime()` which lets you start playback at an arbitrary point in the file (e.g. an argument of `44100` will start one second in).

@antimodular I've added automatic file priming to ofxAudioUnitFilePlayer. This should address the issue where the file player sometimes doesn't start. By default, `play()` will call `prime()`, but you can do it manually as well. Let me know if you ever see the issue again, though.

Closes #8 and closes #5