implemented getPathforFile() helper function to addAudioObject()

Takes an NSString*, returns a std::string. We may want to convert
iOSSonic entirely to Objective-C just for consistency's sake.
removed unused rc barrier_wait return val
replaced time_t with clock_t's for timing

time_t's only do seconds. clock_t's are much more precise.

http://stackoverflow.com/a/12240086/1402368