Fixed various memory leaks especially for mixing (includes recording)

During my tests with this library I found various leaks, especially when using ``AERecorder``. The first two commits (https://github.com/cipherCOM/TheAmazingAudioEngine/commit/9223a8be70ab59f9f0489fbd686426c4de6bc67e, https://github.com/cipherCOM/TheAmazingAudioEngine/commit/1b6c1652b70c1b1054d3fc3d7dc15ff2dd154761) are needed and straightforward.

I had further problems with various ``AVAudioSession`` objects being leaked, mainly in ``audioRouteChangeNotification:``. I could not exactly pinpoint the source but could restrict the amount of bytes leaked by inserting two additional ``autoreleasepool``s. The issue however isn't fixed altogether. I would propose to merge them also as they do no harm but only help. As for the leak I'm almost there to say this is a ``AVFoundation`` bug as the documentation to the corresponding ``AVAudioSessionRouteChangeNotification`` is also wrong. It states the callback happens on the main thread but on iOS 7.1 this happens on a private thread.

BTW awesome library. It was the first library that allowed me to achieve what I wanted. I struggled with EZAudio to get it working but then again this could be due to the missing expertise of audio signal processing.