Remove sp/ prefix from all synthdefs:

* This will aid compatibility with user-made synths
Return to namespaces synth names for mixers and recorder synths:

* This massively reduces the chance that they'll be clobbered by other
  similarly named synths.
* Perhaps we should return to namespaced synth names to remove chance of
  clobbering, or perhaps we should embrace that opportunity?
block synthdef loading with_server_sync rather than nasty sleeps...
add new API fn load_synthdefs: 

* allows user to load their own synthdefs to for triggering with Sonic Pi