rpc_test: Read/write null terminator as part of string
Replace Serial.print with printf in some examples

rpc_test, readback_test, mem_test
sprites: Test collision detection
sprites: Move use of printf until after initialization

Now that it's initialized by the DuinoCube library, any calls to printf()
should follow the call to DC.begin().
sprites: Improve logging of cycle time

Report it infrequently to reduce the amount of logging.
Also, properly initialize the start time to the current time.
Use improved collision interface in sprite demo
Add updated tile reg mapping to defs file
Add new collision table interface to defs file
Updated documentation for release 0.3.0