AEAudioUnitChannel/Filter: Add an initializer with a preInitializeBlock argument

Some audio units properties need to be set before the unit is initialized, this
can now be done in the block passed as preInitializeBlock.
AEAudioUnitFilter: add missing observer for AEAudioControllerDidRecreateGraphNotification
Merge pull request #58 from lijon/auchannelfilter_initblock

Added "preInitializeBlock" initializer for audio unit channel/filter