Minimize direction changes when milling

[eyal0]

When creating a euler path, rather than select the first choice each time, select whichever one keep the milling head travelling in the same direction as much as possible. So if the goal is to mill the red shape, rather than choose a route like the purple one, choose a route like the green one.

This fixes #417

[coveralls]

Coverage increased (+0.07%) to 68.024% when pulling 242962f on eyal0:euler_curves into df24580 on pcb2gcode:master.

[eyal0]

I just looked at the am-test example and it's pretty exciting! Here's what it looks like without this change:

I count 6 right-angle turns and 2 u-turns.

Here it is after this change:

That's 5 right-angles and no u-turns. The amount of total backtracking is identical.

Notice that there is a jump at one point. pcb2gcode calculated that it would be faster to lift, g0, and lower over there rather than backtrack. In other case, though, backtrack was chosen. If I run pcb2gcode with --g0-vertical-speed=5in/min, then I've told pcb2gcode that lifting and lowering the tool with G0 is at 5inch/minute, instead of the default of 50in/min. This is much slower than the default so now backtracking becomes a better option and the jump to complete the shape is removed. Cool!