Use unretract_speed when pressure advance is doing a unretract

When the pressure advance is inserting an filament advance, when there
was no retraction before and ($adv_amnt - $adv_last_amnt) is bigger than
zero it is basicly the same as an unretract, so use the unretract_speed
if it is set.
Fix pressure advance compensated unretract length

Adding the original unretract length together with return value of
extrude($adv_amnt - $adv_last_amnt) basicly means, you are retracting by
two times the retraction length plus the adv_amnt difference, because
the extrude() function already adds the original "E-value" with the
adv_amnt difference.
For example without the fix and 5mm retraction length:

G92 E0 ; reset extrusion distance
G1 X15.000 Y12.640 F15000.000 ; move to first skirt point
G1 E10.06269 F2700.000 ; compensate retraction + pressure advance
G1 X-20.000 Y12.640 E7.25692 F600 ; skirt

The E-value of the first move on the 4th is actually less than the
retraction compensation + pressure advance

With the fix and 5mm retraction length

G92 E0 ; reset extrusion distance
G1 X15.000 Y12.640 F15000.000 ; move to first skirt point
G1 E5.06269 F2700.000 ; compensate retraction + pressure advance
G1 X-20.000 Y12.640 E7.25692 F600 ; skirt

It is extruding correctly again.
typo
Merge pull request #4 from DerFlob/stable

Bug fix from @DerFlob