Add and use get_millis in pid tuning graph

Also, fix simulator not to complain about missing new feature.
Merge 'origin/master' into mine/foo

Conflicts:
	dda.c
	simulator/heater_sim.c
	testcases/run-in-simulavr.sh
	testcases/triangle.gcode
Merge accel_clock from attic
Add testcases for velocity attained

WARNING this script will overwrite your config.h

The velocity-test.sh script will compile the simulator with a set
of canned config parameters and for a range of different ACCELERATION
values.  Then it runs the simulator against a range of different
target speeds, with and without lookahead enabled.  Finally it plots
the results in gnuplot to the screen.

The actual speed is calculated from the trace file using an awk
script to calculate the differential of the position w.r.t time.
Time is on the X axis and is given in minutes.  Speed on the Y
axis is given in mm/min (same as feedrate).
testcases/velocity-test: Vary STEPS_PER_MM

Some values for STEPS_PER_MM can cause overflow errors in the step-time
calculations.  Vary this compile-time constant during the simulation
in velocity-test to help identify where the useful ranges are.
DRY: Reduce duplication in Makefile-AVR

Refactor the "sizes" code into a parametric make function and
reformat it for readability.
Add config.h wrapper to simplify test automation

Test code which wants to customize config.h can do so without
touching config.h itself by wrapping config.h in a macro variable.
Makefile-*: make target makefiles more generic

Allow common makefile variables to be overridden by 'make' caller
so the user's 'Makefile' can be authoritative.

Also, move build products into "USER_CONFIG"-specific folders so
dependencies continue to work as normal.

Example user makefile:

    avr:
            make -f Makefile-AVR F_CPU=20000000L MCU_TARGET=atmega328p
Update URLs for new github location
WIP: My ACCEL_CLOCK testing