Deleted the "endstop" branch.

This is an empty commit just to notify interested parties.
All the interesting bits formerly in the endstop branch
can be found also in the "newdda" branch.
Move ThermistorTable.h out of the repository.

Having this available immediately is nice, having this table
overwritten with each update is a lot worse, though.
Clean up default thermistor tables, analog to config.default.h.

Most likely, these tables don't depend on the type of electronics,
but on the type of thermistor.
Store distances in the TARGET structure always in micrometers.

This is a intrusive patch and for now, it's done for the X axis only.
To make comparison with the former approach easier ...

The advantages of this change:

- Converting from mm to steps in gcode_parse.c and back in dda.c
  wastes cycles and accuracy.

- In dda.c, UM_PER_STEP simply goes away, so distance calculations
  work now with STEPS_PER_MM > 500 just fine. 1/16 microstepping
  on threaded rods (Z axis) becomes possible.

- Distance calculations (feedrate, acceleration, ...) become much
  simpler.

- A wide range of STEPS_PER_M can now be handled at reasonable
  (4 decimal digit) accuracy with a simple macro. Formerly,
  we were limited to 500 steps/mm, now we can do 4'096 steps/mm
  and could easily raise this another digit.

Disadvantages:

- STEPS_PER_MM is gone in config.h, using STEPS_PER_M is required,
  because the preprocessor refuses to compare numbers with decimal
  points in them.

- The DDA has to store the position in steps anyways to avoid
  rounding errors.
Store distances in the TARGET strucure in micrometers for all axes.

This extends the previous commit from X to Y, Z and E.
dda.c: fix a typo in update_current_position().
dda.c: also reset startpoint_steps.E on relative extruder movements.

Thanks to bgamari for identifying this bug.
dda.c: get rid of did_step.

We want to stop stepping as there are no steps left, not one step later.

Accordingly, we get rid of a small pause between two movements and
also have to decelerate one step earlier.
Get rid of defered enabling of the step interrupt again.

This makes the code cleaner and the reduction of code
probably easily compensates for keeping global interrupts
enabled for a bit longer. Talked to macscifi about this.

Saves about 300 bytes of binary size.
timer.c: get rid of explicitely disabling the step timer.

This is no longer neccessary, as the step timer is not designed to
fire always exactly once.

Saves 54 bytes binary size.
timer.c: simplify clock counting a bit.

Nothing fancy, but it saves 8 bytes, so likely 8 cycles.
configs: make F_CPU UL instead of L.
home.c: fix a typo.
Configuration samples: don't use preprocessor math for STEPS_PER_M.

Since the introduction of STEPS_PER_M instead of STEPS_PER_MM
it's no longer allowed. Unfortunately.
Make fast PWM optional.

In some cases the heater switching MOSFETs overheat and using a lower
PWM frequency is advised. See also:
http://reprap.org/wiki/Gen7_Research#Heat_vs._PWM_Frequency
gcode_process.c: add notes about M82 and M83.
dda.c: re-introduce ACCELERATION_TEMPORAL.

The implementation is slightly different this time, as it's not
using these famous bresenham algorithms. The intention is to
allow axis-independent movements, as it's required for
EMC-quality look-ahead.
Make the step timer fit for very short delays.

This isn't the magic patch which suddenly allows us to run at
arbitrary high step rates, but can deal with short bursts of
very short delays only. Typical for a 45 deg move when using
ACCELERATION_TEMPORAL.

The implementation simply extends the impossible small delay.
To keep overall timing promises regardless, the extra is stored
and compensated for when a longer delay request comes in.

Wrapped all this in #ifdef ACCELERATION_TEMPORAL, as this adds
about 300 bytes of binary size, so it likely slows down the
setTimer() code a bit and non-temporal algorithms are expected to
never request an unreasonable short delay.
Save status register on each interrupt.

Costs 28 bytes binary size and two CPU cycles per interrupt,
at the advantage of getting rid of possible well hidden bugs.
dda.c: get rid of a unused variable warning.