Iterator loops replaced with C++11 loops.
Fixed clone() methods to return an ExtrusionEntity*.
PerimeterGenerator now uses move semantics on ExtrusionEntity a little
bit more.
The gap fill was disabled for zero infill.
Now the gap fill is enabled in between the perimeters, but disabled between
the inner-most perimeter and infill in case the infill is set to zero.
Also in case there are multiple infill regions inside a perimeter,
the mutliple infills are considered as non-zero if at least one infill
is non-zero, therefore the gap fill will be added inside the inner-most
perimeter.
