Move endstop handling into a time-based procedure.

Before, endstops were checked on every step, wasting precious time.
Checking them 500 times a second should be more than sufficient.

Additionally, an endstop stop now properly decelerates the movement.
This is one important step towards handling accidental endstop hits
gracefully, as it avoids step losses in such situations.
dda.c: fix ACCELERATION_RAMPING #ifdefs.
config.teensy.h: test settings on a Wallace
Try cyberwizzards lookahead on teensypp
config.??.h: test for LOOKAHEAD + ACCELERATION_RAMPING and comment an unused variable
dda_lookahead.h: Allow non-LOOKAHEAD compilation
dda.*: drop lead-axis tecking
dda.c: Replace mistakenly elided bracket
Port to Teensy3.0 (ARM Cortex-m4 mk20dx128)
mk20dx128.ld: attempt to add .eeprom section
merge with Triffid/Teacup_Firmware/Gen7
dda.c and dda_queue.c: replace SREG/cli/sei
README: add note on teensy3.0 development branch
README, Makefile-teensy3: Note differences for ARM compilation
Clean out some compiler warnings
Makefile-teensy3: clean up spurious generation of .d files
mendel.c,arduino_teensy3.h,analog_ARM.c: Enable ADC interrupt service
heater.c: enable PWM on pwm pins
heater.c: expand 8-bit PWM value to full range of PWM
README: Note that ADC+PID+PWM now works