Switch to non-blocking implementation of DSLogger

Hi Dustin, -- I hope I haven't overwhelmed you with my recent requests...

With these changes I propose a new, non-blocking (lock-free) implementation of **DSLogger** -- the implementation is based on the great GCD mechanism. With this approach, the threads calling **DSLog** are never blocked -- even if some other threads call it too and even if the whole buffer content is being prepared at the very same moment. Testing it for a few days, there is definitely no performance degradation (at least).