Max velocity axes independence installed. Fixed intermittent slow trailing steps. Timer0 disable fix.

- Maximum velocity for each axis is now configurable in settings. All
rapids/seek move at these maximums. All feed rates(including rapids)
may be limited and scaled down so that no axis does not exceed their
limits.

- Moved around auto-cycle start. May change later, but mainly to ensure
the planner buffer is completely full before cycle starting a streaming
program. Otherwise it should auto-start when there is a break in the
serial stream.

- Reverted old block->max_entry_speed_sqr calculations. Feedrate
overrides not close to ready at all.

- Fixed intermittent slow trailing steps for some triangle velocity
profile moves. The acceleration tick counter updating was corrected to
be exact for that particular transition. Should be ok for normal
trapezoidal profiles.

- Fixed the Timer0 disable after a step pulse falling edge. Thanks
@blinkenlight!

config.h

@@ -122,7 +122,7 @@
 // frequency goes up. So there will be little left for other processes like arcs.  
 //   In future versions, more work will be done to increase the step rates but still stay around
 // 20kHz by performing two steps per step event, rather than just one.
-#define ISR_TICKS_PER_SECOND 30000L  // Integer (Hz)
+#define ISR_TICKS_PER_SECOND 20000L  // Integer (Hz)
 
 // The Ranade algorithm can use either floating point or long integers for its counters, but for 
 // integers the counter values must be scaled since these values can be very small (10^-6). This